DETAILS  OF  CYANIDE  PRACTICE 


McGraw-Hill  Book  Company 


Electrical  World         The  Engineering  and  Mining  Journal 
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R  ailway  Age  G  aze  tt<?  American  Machinist 

Signal  Elngin<?9r  American  Engineer 

Electric  Railway  Journal  Coal  Age 

Metallurgical  and  Chemical  Engineering  P  o  we  r 


DETAILS  OF 
CYANIDE  PRACTICE 


BY 
HERBERT  A.  MEGRAW 

MINING   AND   METALLURGICAL   ENGINEER.    MEMBER   OF   THE  AMERICAN 

INSTITUTE  OP  MINING  ENGINEERS.  MEMBER  OF  EDITORIAL  STAFF 

OF  THE  "ENGINEERING  AND  MINING  JOURNAL."  AUTHOR 

OF  "PRACTICAL  DATA  FOR  THE  CYANIDE  PLANT." 


FIRST   EDITION 


McGRAW-HILL  BOOK  COMPANY,  INC. 
239  WEST  39TH  STREET,  NEW  YORK 

6  BOUVERIE  STREET,  LONDON,  E.  C. 
1914 


T/y 
A/.-f 


COPYRIGHT,  1914,  BY  THE 
McGRAW-HiLL  BOOK  COMPANY,  INC. 


0 


THE. MAPLE- PRESS- YOKK. PA 


PREFACE 

The  chapters  presented  in  this  book  are  the  record  of  an  investigative 
tour  of  the  principal  cyaniding  mills  of  North  America,  taken  with  the 
idea  of  presenting  detailed  facts  about  the  present  state  of  the  process  to 
the  readers  of  the  Engineering  and  Mining  Journal,  in  which  periodical 
they  have  already  appeared.  Actual  description  has  been  considered  of 
less  importance  than  discussion  and  correlation  of  facts  gathered  from 
widely  different  places,  the  intention  having  been  to  present  facts  and 
personal  opinions,  clearly  indicated  as  such,  in  a  form  calculated  to 
inform  the  profession  in  general  and  to  promote  discussion  respecting 
details  involving  diverging  practice.  Some  pages  of  the  discussion 
elicited  have  been  included  as  illuminating  the  subjects. 

One  article,  that  constituting  Chapter  II,  has  been  included  that  I 
did  not  write.  Although  I  had  visited  and  studied  the  plant  discussed, 
and  had  formulated  a  paper  treating  of  its  interesting  points,  the  de- 
scription and  explanation  submitted  by  Mr.  R.  B.  Watson,  manager  of 
the  Nipissing  Mining  Co  ,  was  used,  a  manager  being  more  thoroughly 
familiar  with  his  own  plant  than  anyone  else  could  be.  Some  points 
which  particularly  attract  the  attention  of  the  outsider,  however,  were 
mentioned  in  my  discussion  of  the  subject,  and  these  are  to  be  found 
constituting  Chapter  III.  Those  points  thoroughly  covered  by  Mr. 
Watson  have  been  removed  from  my  article,  which  makes  it  somewhat 
disjointed,  but  anyone  reading  the  two  chapters  will  be  at  no  trouble  to 
understand  the  references 

The  original  itinerary  of  this  technical  journey  included  a  visit  to  the 
important  plants  of  Mexico,  but  the  condition  of  affairs  in  that  country 
when  the  trip  was  undertaken,  made  it  necessary  for  that  part  to  be 
postponed  until  such  time  as  there  will  be  more  to  study  and  greater 
safety  in  traveling. 

Any  benefit  which  the  profession  may  derive  from  the  matter  pre- 
sented is  to  be  credited  to  Mr.  Walter  Renton  Ingalls,  Editor  of  the 
Engineering  and  Mining  Journal,  to  whose  energy  and  progressive 
spirit  the  undertaking  is  due. 

H.   A.  MEGRAW. 
NEW  YORK, 
February  10,  1914. 


CONTENTS 

CHAPTER  PA OR 

PREFACE      v 

I.  THE  COBALT  DISTRICT,  ONTARIO 1 

II.  THE  NIPISSING  HIGH-GRADE  MILL,  COBALT. 14 

III.  THE  NIPISSING  HIGH-GRADE  MILL,  COBALT,  Concluded  . 23 

IV.  THE  HOLLINGER  MILL,  PORCUPINE,  ONTARIO 28 

V.  THE  DOME  MILL,  SOUTH  PORCUPINE,  ONTARIO 39 

VI.  PRACTICE  IN  THE  BLACK  HILLS,  SOUTH  DAKOTA 50 

VII.  THE  LIBERTY  BELL  MILL,  TELLURIDE,  COLORADO 67 

VIII.  PRACTICE  AT  CRIPPLE  CREEK,  COLORADO 79 

IX.  CONTINUOUS  DECANTATION  OF  SLIME 96 

X.  PRACTICE  AT  TONOPAH 102 

XI.  PRACTICE  AT  TONOPAH,  Continued 114 

XII.  PRACTICE  AT  TONOPAH,  Concluded 129 

XIII.  THE  NEVADA  HILLS  MILL  AT  FAIRVIEW 153 

XIV.  PRACTICE  AT  THE  NEVADA  WONDER  MILL 161 

XV.  METHODS  AT  REPUBLIC 168 

XVI.  THE  MILLS  OF  GRASS  VALLEY,  CALIFORNIA      177 

XVII.  THE  BLACK  OAK  PLANT,  CALIFORNIA 188 

XVIII.  THE  GOLD  ROAD  MILL,  ARIZONA 195 

XIX.  Two  ARIZONA  MILLS 203 

INDEX  .                                                                                                                .  213 


Vll 


DETAILS  OF  CYANIDE  PRACTICE 

CHAPTER  I 
THE  COBALT  DISTRICT,  ONTARIO 

The  mining  district  of  Cobalt  in  Ontario,  Canada,  is  one  of  the 
newer  areas  in  which  economic  mining  has  proved  possible.  In  common 
with  most  new  camps  the  first  output  was  high-grade  material,  but  it 
differs  from  many  in  the  fact  that  the  average  content  of  the  mineral 
has  continued  up  to  this  time  to  be  higher  than  is  usually  the  case  when 
work  has  been  carried  to  the  extent  that  it  has  here.  The  valuable 
metal  is  silver  which  occurs  native  and  in  combination.  It  is  found 
as  sulphide,  arsenide,  antimonide  and  in  combination  with  manganese, 
nickel  and  cobalt.  Small  proportions  of  copper  are  also  found  in  some 
of  the  ores  and  in  at  least  one,  the  Nipissing,  mercury  has  been  definitely 
proved  to  exist.  In  regard  to  the  latter  many  engineers  are  skeptical, 
having  never  encountered  any  indications  of  it  even  in  the  higher  grade 
mineral  but  the  work  of  the  metallurgists  who  experimented  upon  the  ores 
of  the  Nipissing  mine  has  been  sufficient  for  them  to  state  positively  that 
a  small  quantity  of  mercury  does  exist  as  a  mercury-silver  amalgam. 

Character  of  Ore. — The  ores  occur  in  narrow  fissures  in  a  country 
rock  of  fine-grained  conglomerate,  slate  or  diabase.  They  are  all  hard 
and  tough  and  are  not  easily  ground  to  a  fine  state.  The  experience 
of  the  camp  has  been  that  when  the  ores  are  ground  extremely  fine,  even 
to  a  point  where  they  will  pass  a  200-mesh  screen,  the  product  is  largely 
granular  and  has  the  property  of  settling  quickly,  the  light,  flocculent 
product  generally  called  true  slime  being  conspicuous  for  its  rarity.  In 
addition  to  this,  the  mineral  is  extremely  heavy,  so  that  the  difficulty 
of  obtaining  a  product  which  may  be  treated  as  a  total  slime  is  greater 
than  is  usually  the  case  with  silver  ores.  The  power  and  care  required 
to  keep  it  in  suspension  is  comparatively  great. 

Most  of  the  mills  in  the  Cobalt  camp  are  concentrators  and  due  to 
the  character  of  the  mineral,  which  is  favorable  to  concentration,  good 
results  have  been  obtained  from  this  system  of  treatment.  The  silver 
occurs,  as  has  been  mentioned,  in  heavy  minerals  which  give  up  a  large 
percentage  of  their  silver  content  on  the  concentrating  table.  Never- 
theless, the  tailing  discarded  by  the  concentrating  mills  contains  sufficient 
silver  so  that  its  extraction,  or  partial  extraction,  by  cyanidation  while 
the  pulp  is  in  motion  through  the  mill,  might  be  expected  to  repay  more 

1 


2  DETAILS  OF  CYANIDE  PRACTICE 

than  the  additional  expense  of  cyanide  treatment.     It  is  altogether  likely 
that  the  average  tailing  from  a  concentrator  treating  25-  to  35-oz.  ore 
will  contain  not  less  than  five  to  eight  ounces  of  silver  per  ton.     Most 
of  the  operators  will  not  admit  that  the  tailing  runs  so  high,  but  a  con- 
sideration of  the  conditions,  together  with  some  frank  conversation  with 
operators  leads  me  to  believe  that  these  figures  do  not  overstate  the  case. 
If  this  is  true,  then  cyanide  treatment  of  the  table  tailing  should  be 
economical  in  view  of  the  fact  that  most  of  the  work  has  already  been 
done  on  the  ore  and  the  additional  cost  of  regrinding  plus  the  cost  of 
chemicals  and  labor  would  be  more  than  met  by  the  additional  silver 
recovered.     I  have  already  said  that  the  ore,  even  when  finely  ground, 
is  difficult  to  agitate,  but  this  does  not  conflict  with  the  idea  that  cyanida- 
tion  would  be  economical,  because  it  is  conceivable  that  the  fine  sand 
might  be  leached  at  a  cost  which  would  be  less  than  that  of  agitating 
it.     I  am  unable  to  discover  that  any  exhaustive  experiments  have  been 
carried  out  on  leaching  a  concentrate  tailing,  but  I  do  not  know  of  any 
good  reason  why  it  would  not  be  possible,  particularly  in  those  mills  where 
concentration  has  already  been  installed.     It  may  be  true  that  a  new  mill 
designed  for  cyaniding  the  entire  product  would  give  the  most  economical 
result  eventually,  but  that  is  no  reason  why  mills  now  operating  as  con- 
centrators could  not  increase  their  profits  by  cyaniding  the  table  tailing 
in  some  way.     The  analysis  of  the  ores  support  this  view  in  so  far  as  show- 
ing that  the  higher  grade  ores  contain  most  of  the  elements  considered 
rebellious,  while  the  lower  grade  ores  are  comparatively  free  from  them, 
making  cyanidation  cheaper  on  the  low-grade  minerals.     In  the  accom- 
panying table  is  presented  a  statement  of  the  comparative  content  of  the 
extraordinary  elements  in  Cobalt  ores  of  various  grades,  A  representing 
the  high-grade,  B  the  medium-grade  and  C  the  low,  or  milling-grade  ores. 


TYPICAL  ANALYSES  OF  COBALT  ORES  OF  DIFFERENT  GRADES 


A 

B 

C 

SiO2  

4  51 

2  88 

Fe  

2  34 

2  80 

7  00 

CaO.. 

9  05 

10  00 

Q  00 

A12O3  

1  42 

0  87 

15  00 

MgO  

6  22 

7  13 

Ni  

6  62 

8  78 

Co  

7  11 

8  42 

As.. 

29  88 

34  48 

0  <=;o 

Ag.  (oz.  per  ton)  

4786.1 

2014.00 

71.27 

Present  Cyaniding  Mills. — Four  of  the  Cobalt  companies  have  under- 
taken cyanidation  in  some  form  and  degree.  These  are  the  Nipissing 
Mining  Co.,  the  Buffalo  Mines  Co.,  the  Dominion  Reduction  Co.  and 


THE  COBALT  DISTRICT,  ONTARIO  3 

the  O'Brien. 1  The  Buffalo  mill2  treats  partly  by  concentration  and  partly 
by  cyanidation.  The  ore  is  broken  through  crusher  and  rolls  and  is 
partly  reground  in  a  high-speed  Chilean  mill,  the  crushing  and  grinding 
being  done  in  water.  The  slime  is  separated,  thickened  in  a  Dorr 
thickener  and  cyanided  in  Pachuca  tanks,  the  sand  being  concentrated 
in  water.  The  tailing  is  stacked  and  may  be  retreated  at  some  future 
time.  Of  the  sand  reground  in  the  Chilean  mill,  only  about  5%  is  slimed, 
the  total  quantity  of  slime  cyanided  being  only  18  to  20%  of  the  total 
ore  crushed.  The  —  200-mesh  product  is  considered  as  slime  and  it  is 
largely  a  granular  product,  there  being  very  little  colloid  in  it. 


NEW  LOW  GRADE-MILL  AT  THE  NIPISSING. 

The  mill  of  the  Dominion  Reduction  Co.,  formerly  known  as  the 
Nova  Scotia,3  crushes  with  stamps  in  cyanide  solution.  The  mill  was 
erected  for  cyanide  treatment  alone,  small  allowance  being  made  for 
concentration,  but  having  lately  entered  the  custom-milling  field  it  was 
found  necessary  to  reconstruct  the  mill,  due  to  the  different  character 
of  ore  to  be  treated,  and  this  work  is  now  in  progress,  a  system  of  careful 
concentration  being  interposed  between  the  grinding  and  cyaniding 
departments.  The  regrinding  in  this  mill  is  done  in  tube  mills.  Under 
the  reformed  system  the  pulp  will  be  passed  through  a  Callow  screen 
with  a  30-  or  40-mesh  wire  cloth,  the  under  size  going  to  sand  tables  and 

1  The  O'Brien  is  the  property  of  one  man,  is  not  incorporated  and  is  known  simply 
as  the  O'Brien  mine. 

2  Eng.  and  Min.  Journ.,  Aug.  3,  1912. 
3Eng.  and  Min.  Journ.,  June  8,  1912. 


4  DETAILS  OF  CYANIDE  PRACTICE 

the  oversize  being  reground  and  returned  to  the  Callow  screen.     Thus 
all  the  pulp  will  have  to  pass  the  screen  eventually.     From  the  concen- 


MILL  OF  THE  DOMINION  REDUCTION  CO. 


THE  O  BRIEN  MILL. 


trators  it  will  be  sent  to  a  modified  Dorr  classifier  where  the  sand  will  be 
drained  and  taken  out,  receiving  a  wash  of  cyanide  solution  in  the  classi- 
fier, and  will  then  be  discarded.  The  slime,  which  carries  the  greater 


THE  COBALT  DISTRICT,  ONTARIO  5 

part  of  the  valuable  content,  will  be  subjected  to  cyanidation  by  agitation. 

At  the  O'Brien  mill  the  ore  is  crushed,  jigged  and  stamped,  jigging 
being  done  in  water  and  stamping  in  solution.  Recrushing  is  done  in 
Hardinge  conical  mills.  Concentration  is  practised,  but  to  a  less  extent 
than  usual  in  the  Cobalt  mills,  the  pulp  receiving  only  one  passage  over 
the  tables.  The  reground  pulp  is  treated  as  slime,  about  75%  of  it 
passing  a  200-mesh  screen. 

New  Mill  for  Low-grade  Ores. — The  Nipissing  company  has  under 
construction  a  mill  for  treating  its  so-called  low-grade  ores  which  assay 
from  25  to  35  oz.  silver  per  ton.  This  mill,  which  is  scheduled  to  begin 
operations  in  November  of  the  present  year,  will  be  a  purely  cyaniding 
one,  the  only  variation  being  a  jigging  of  the  crushed  ores.  The  high- 
grade  rock  will  be  removed  on  a  picking  belt,  and  this,  together  with  the 
rich  jig  product,  will  be  treated  in  the  present  high-grade  mill  which  will 
be  maintained  in  operation,  as  at  present,  for  taking  care  of  the  high- 
grade  mine  ores  and  the  rich  product  obtained  by  the  picking  and  jigging 
methods.  There  will  be  no  concentration,  no  tables  or  vanners  having 
been  included  in  the  design  of  the  mill.  There  will  be  40  stamps  of  1500 
Ib.  each  and  four  tube  mills,  6 J  X  20  ft.  Dorr  classifiers  are  to  be  used 
for  removing  the-  sand  which  will  be  tube  milled,  the  entire  product  being 
treated  by  agitation  as  slime.  Zinc-dust  precipitation  will  be  used,  the 
standard  Merrill  triangular  press  being  installed  to  collect  the  precipitate. 
The  standard  Butters  filter  will  be  used  for  filtering  the  residue. 

The  agitation  tanks  to  be  used  have  the  flat  bottoms  and  mechanical 
agitators  usually  employed  by  the  Butters  engineers.  The  tanks  are 
fitted  with  decanters  and  treatment  will  be  by  the  intermittent  or  charge 
system.  It  is  stated  that  these  agitators,  while  not  perfect,  will  give 
as  good  results  as  can  be  obtained  by  the  newer  forms  at  costs  which  are 
entirely  satisfactory  in  comparison.  It  is  noteworthy  that  in  the  face 
of  the  more  modern  forms  of  agitators,  for  which  higher  efficiency  and 
lower  costs  have  been  claimed,  the  older  type  of  agitators  are  being  in- 
stalled in  this  modern  mill,  and  also  that  in  spite  of  the  advantages 
claimed  for  continuous  processes,  the  charge  system  will  be  employed, 
and  all  of  this  sanctioned  by  a  body  of  engineers  which,  while  not  in- 
fallible, is  fully  competent  to  weigh  the  advantages  of  all  classes  of 
machinery  and  make  use  of  the  best.  The  difference  of  opinion  is  re- 
markable and  goes  to  show  that  neither  form  of  machinery  nor  method 
has  been  able  to  demonstrate  so  clearly  its  superiority  as  to  make  it 
universally  accepted.  It  seems  almost  certain  that  one  or  the  other  of 
two  forms  of  machinery,  or  even  of  processes,  must  be  the  better,  the  only 
explanation  of  the  varying  opinions  being  that  different  ores  must  give 
different  results  when  handled  by  the  different  processes.  Neither  is 
the  personal  equation  to  be  eliminated,  for  operators  accustomed  to  a 


6  DETAILS  OF  CYANIDE  PRACTICE 

certain  class  of  machinery  and  certain  methods  of  working,  do  not  re- 
linquish them  with  facility,  and  it  is  often  the  most  difficult  thing  in  the 
world  to  convince  an  operator  that  there  is  some  better  way  of  doing 
things  than  the  particular  one  he  is  using. 

Primary-crushing  Devices. — -The  persistent  use  of  stamps  for  re- 
ducing rock  to  various  degrees  of  fineness  is  an  example  of  the  general 
disinclination  to  dispense  with  a  form  of  machinery  which  has  been  in 
use  a  considerable  length  of  time  and  which,  all  things  considered,  has 
given  a  vast  amount  of  satisfactory  service.  It  is  difficult  to  censure 
the  profession  for  sticking  to  a  device  which  has  served  well,  but  it  is 
nevertheless  true  that  circumstances  alter  cases,  and  a  machine  ought 
to  be  considered  in  the  light  of  the  work  it  is  expected  to  do.  When  the 
primary  object  was  to  crush  ore  to  a  point  necessary  to  liberate  its  mineral 
and  make  it  susceptible  to  amalgamation,  then  the  stamp  was  a  good 
medium  to  use.  It  did  the  work  at  one  step,  and  had  advantages  for 
amalgamation.  It  made  comparatively  little  slime,  and  this  was  an 
advantage,  particularly  in  the  days  when  cyanidation  was  first  used  and 
leaching  was  the  accepted  practice.  The  slime  was  a  nuisance  and  was 
hard  to  handle  and  therefore  any  machine  which  produced  a  minimum 
of  it  was  considered  efficient.  With  the  advent  of  the  total-slime  treat- 
ment it  would  seem  likely  that  some  other  machine  might  be  used  which 
would  do  the  work  of  reducing  ores  to  a  fine  state  quicker  and  cheaper, 
it  not  being  necessary  to  keep  the  production  of  slime  down  to  a  minimum. 

In  spite  of  this  circumstance,  the  stamp  continues  to  be  the  standard 
machine  for  primary  crushing,  although  there  is  little  doubt  but  that 
some  other  form  of  machine  would  be  more  economical  in  use.  This  con- 
dition exists  all  over  the  mining  world  and  Cobalt  is  no  exception.  In 
preparing  ores  for  cyanidation  there  is  no  other  object  but  to  crush  them 
to  the  required  point,  usually  a  total  slime,  in  the  quickest  and  cheapest 
way.  Yet  most  mills  use  stamps  for  the  purpose  and  in  Cobalt  the  stamp 
is  used  except  in  one  instance,  the  Buffalo  mill.  All  over  the  world  in- 
vestigations are  being  made  on  this  detail  and  in  some  cases  the  stamps 
have  been  displaced  with  distinctly  good  results.  I  think  that  if  some 
trials  were  ma'de  on  the  Cobalt  ores  these  economies  would  be  manifested 
there  also.  The  Cobalt  ores  are  hard,  but  not  too  hard  to  be  crushed 
through  rolls,  and  a  stage  crushing  through  crushers,  rolls  and  tube  mills 
would  seem  to  indicate  the  most  economical  method.  It  is  to  be  hoped 
that  the  system  will  be  given  thorough  trials  on  the  ores  of  the  camp  for 
such  a  system  would  remove  many  vexations  in  addition  to  introducing 
economies. 

Regrinding  the  Ores. — Tube  mills  are  used  for  regrinding  in  the 
majority  of  the  mills  in  the  Cobalt  camp.  The  Nipissing,  new  and  old 
mills,  the  Dominion  and  the  O'Brien  mills  have  installations  of  them 


THE  COBALT  DISTRICT,  ONTARIO  7 

while  the  Buffalo,  as  has  already  been  mentioned,  uses  a  high-speed 
chilean  mill.  There  is  no  data  at  hand  for  comparing  the  results  of  the 
two  forms  of  machines,  but  it  is  probable,  on  this  ore,  that  the  tube  mill 
will  give  more  consistent  results.  The  Hardinge  conical  mill,  as  used 
at  the  O'Brien  mill,  has  not  proved  to  be  altogether  satisfactory.  The 
mill  is  apparently  good  as  a  granulator,  reducing  the  rock  to  a  state  of 
medium  fine  sand  with  efficiency,  but  it  does  not  grind  fine  enough. 
In  this  regard  the  ordinary  tube  mill  is  more  satisfactory.  It  is  possible 
to  slime  the  rock  in  the  Hardinge  mill,  but  to  do  so  would  reduce  its 
capacity  so  far  as  to  remove  it  from  consideration  as  an  economical 
machine.  It  is  possible  that  the  hardness  of  the  ore  has  something  to  do 
with  this  result  for,  as  has  been  mentioned,  the  ore  is  extremely  difficult 
to  slime. 

Methods  of  Agitation. — Pachuca  tanks  are  used  generally  for  agita- 
tion except  in  the  cases  of  the  Nipissing  company,  and  at  the  Dominion 
mill  where  Trent  agitators  are  at  present  in  use.  The  Pachuca  tanks  used 
in  the  district  are  somewhat  modified,  not  conforming  exactly  with  the 
standard  sizes  and  proportions  used  in  Mexico  and  in  the  United  States, 
the  difference  being  that  they  are  not  so  high  in  proportion  to  the  diam- 
eter. While  this  reduces  the  capacity,  there  is  no  effect  upon  the  agita- 
tion except,  perhaps,  that  it  requires  less  power  to  accomplish  the  desired 
result.  The  Parral  type  of  agitator  is  to  have  a  trial  in  the  camp  at  thf 
new  high-grade  mill  of  the  Buffalo  company,  now  under  construction. 
This  plant,  which  is,  in  the  main,  similar  to  the  corresponding  plant  of 
the  Nipissing  company,  contains  some  details  which  are  considered  to  be 
improvements  and  will  be  referred  to  later.  In  view  of  the  fact  that  it 
has  been  difficult  for  metallurgists  to  agree  upon  the  most  satisfactory 
tank  for  agitation,  it  will  be  interesting  to  have  a  comparative  exhibition 
of  the  three  classes,  Pachuca,  Parral  and  mechanical,  at  work  in  the  same 
camp  and  upon  the  same  material.  It  is  to  be  hoped  that  full  details  of 
the  results  will  be  obtainable. 

Use  of  Chemicals. — Lime  in  some  form  is  used  at  all  the  cyanide  mills 
in  the  camp.  At  the  O'Brien  mill  the  dry  lime  is  added  in  the  batteries 
and  in  addition  caustic  soda  is  used  in  the  solutions.  Caustic  soda  is 
not  usually  considered  a  good  agent  to  use  for  securing  alkalinity  as  it 
tends  toward  dissolving  some  compounds  which  would  be  better  left 
untouched.  There  are,  however,  some  cases  where  its  use  is  justified 
in  spite  of  its  chemical  conduct  and  its  cost.  The  alkalinity  at  the 
O'Brien  is  carried  at  3  Ib.  in  terms  of  NaOH.  At  the  Dominion  mill  dry 
lime  is  added  to  the  ore  going  to  the  mill  at  the  rate  of  about  6  or  7  Ib. 
per  ton.  The  lime  is  in  fine  lumps,  not  powdered,  as  it  is  desired  to  take 
advantage  of  the  alkalinity  developed  by  the  slow  solution  of  the  lumps. 

At  the  Buffalo  mill  lime  is  added  in  powdered  form  to  the  treatment 


8  DETAILS  OF  CYANIDE  PRACTICE 

tanks  at  the  rate  of  7  to  8  Ib.  per  ton  of  dry  ore.  The  addition  of  the 
lime  to  the  treatment  tanks  in  this  case  is  due  to  the  fact  that  the  pulp 
here  meets  the  cyanide  solution  for  the  first  time,  the  preceding  opera- 
tions being  carried  out  in  water. 

It  will  be  noted  that  there  is  a  great  deal  of  difference  in  the  manner  of 
using  lime  in  this  district,  every  operator  having  some  special  ideas  of 
the  most  efficient  way.  Naturally,  the  lime  will  be  used  in  the  way  which 
seems  to  be  best  adapted  to  each  particular  ore,  but  in  this  case  the  use  of 
it  typifies  the  state  of  cyanide  practice  in  the  district.  It  is  as  yet  unset- 
tled, and  the  best  methods  for  economical  results  have  not  yet  been  defi- 
nitely determined. 

The  use  of  lead  salts  is  varied  and  spasmodic.  There  seems  to  be  no 
definite  information  as  to  whether  or  not  they  are  required  and  their  use, 
even  in  the  same  mill,  is  not  systematized.  The  general  tendency 
seems  to  be  to  dispense  with  their  use.  At  the  Buffalo  mill  it  is  claimed 
that  there  is  no  noticeable  difference  in  results  whether  the  lead  salts  are 
used  or  not,  and  the  same  is  true  at  the  O'Brien  mill.  Solution  analyses 
at  the  O'Brien  have  convinced  the  operators  that  the  sulphur,  which 
exists  in  appreciable  quantity  in  solution,  is  in  the  form  of  sulphocyanide. 
This  would  indicate  that  the  soluble  sulphides,  which  are  undoubtedly 
first  formed,  have  been  transformed  by  action  of  the  metal  elements 
extracted  from  the  ore  and  subsequently  by  the  cyanide  solutions  them- 
selves. If  this  is  true  it  shows  that  a  portion  of  the  cyanide  is  removed 
from  its  primary  object  of  dissolving  silver  and  is  tied  up  with  the  sulphur 
forming  a  compound  which,  while  having  a  certain  solvent  action 
upon  silver,  would  certainly  not  be  intentionally  formed  for  such  pur- 
pose. It  would  apparently  be  more  economical  to  remove  the  sulphur 
definitely  from  the  solution,  if  possible,  leaving  the  cyanides  free  to  carry 
out  their  designed  function.  It  is  probably  true  that  the  Cobalt  ores  carry 
elements  which  themselves  form  insoluble  sulphides  with  the  dissolved 
sulphur  and  remove  them,  at  least  partially,  from  the  solutions. 

The  consumption  of  cyanide  is  much  the  same  in  all  the  mills,  leav- 
ing out,  of  course,  the  Nipissing  high-grade  mill,  which  is  not  to  be  con- 
sidered with  the  mills  treating  large  quantities  of  lower  grade  ores. 
From  3  to  6  Ib.  of  KCN  per  ton  of  ore  treated  is  the  average  con- 
sumption, when  treating  ores  running  from  20  to  35  oz.  of  silver.  At 
the  O'Brien  mill  the  consumption  for  the  year  1911  was  5  Ib.  KCN  per 
ton  of  ore  treated,  while  for  the  present  year  the  records  up  to  date  show 
a  consumption  of  3.8  Ib.  per  ton.  At  the  Buffalo  and  Dominion  mills 
the  consumption  will  average  about  the  same  and  at  the  Nipissing  high- 
grade  mill  the  figures  are  reserved  from  publication,  but  the  consump- 
tion is  probably  high. 

Metal  Recoveries. — The  percentage  recovery  of  silver  seems  to  vary 


THE  COBALT  DISTRICT,  ONTARIO  9 

a  great  deal  in  the  different  mills  of  the  camp.  From  80  to  90%  is  about 
the  saving  that  is  attained  by  cyanidation,  except  in  the  Nipissing  mill. 
In  the  latter  a  saving  is  made  by  amalgamation  of  97%  and  in  the  subse- 
quent cyanidation  a  small  additional  saving  is  made.  The  head  content 
of  silver  at  this  mill  runs  about  2500  oz.  silver  and  the  tailing  is  said  to  be 
as  low  as  20  to  30  oz.  per  ton.  The  extraordinary  percentage  of  extraction 
is,  of  course,  accounted  for  by  the  richness  of  the  ore  treated.  At  the 
O'Brien  mill  concentration  and  cyanidation  reduce  20-oz.  heads  to  a 
tailing  which  averages  1  to  1.5  oz.  per  ton  in  silver.  At  the  Buffalo  mill 
where  only  the  slime  is  cyanided  the  extraction  on  the  material  is  from 
80  to  85%,  but  this  is  by  the  cyanide  treatment  alone  and  no  comparisons 
can  be  made  between  this  result  and  the  results  obtained  at  the  other 
mills  in  the  district.  At  the  Dominion  mill  no  extraction  results  are 
obtainable  at  this  time  due  to  the  fact  that  a  change  of  method  is  being 
made,  and  while  the  mill  is  actually  in  operation,  the  newer  method  is 
not  yet  fully  installed  and  no  statement  can  be  made. 

It  is  generally  conceded  that  the  percentage  extraction  depends  largely 
upon  the  fineness  to  which  the  ore  is  ground,  the  finer  grinding  giving  a 
better  recovery  of  metal  than  the  coarser.  This  is  to  be  expected  in  view 
of  the  density  of  the  ore  which  does  not  permit  penetration  of  the  cyanide 
solutions  into  the  ore  particles  and  requires  extreme  subdivision  in  order 
to  expose  the  maximum  amount  of  the  silver  to  the  action  of  solutions. 

Temperature  has  also  a  great  effect  upon  extractions.  In  the  winter 
season  the  cold  solutions  lower  extractions  to  a  large  extent  and  it  is  often 
necessary  to  warm  the  solutions  in  order  to  obtain  normal  results.  At 
the  Buffalo  mill  steam  coils  are  placed  in  the  treatment  tanks  in  order  to 
bring  the  solutions  up  to  normal  efficiency.  In  this  case  the  warming  is 
particularly  necessary,  for  there  is  a  constant  introduction  of  cold  water 
into  the  treatment  circulation  from  the  grinding  department.  At  other 
mills  where  the  circulation  is  entirely  cyanide  solution,  a  thorough  heating 
of  the  mill  during  the  winter  season  suffices  to  keep  the  solutions  active. 

Precipitation  Systems. — The  precipitant  most  favored  is  zinc  dus+ 
which  will  ultimately  be  used  in  practically  all  the  mills.  At  the  Buffalo 
mill  the  precipitation  is  at  present  by  zinc  shavings,  three  wooden  boxes 
of  the  ordinary  type  being  used.  At  the  Dominion  mill  zinc  dust  is  used 
and  the  regular  Merrill  triangular  filter  press  is  installed  for  collecting  the 
precipitate.  The  consumption  is  in  this  case  If  of  zinc  to  one  of  silver 
precipitated,  by  weight.  At  the  new  mill  of  the  Nipissing  company  the 
same  process  will  be  used  and  the  resultant  precipitate  will  be  sent  to  the 
refinery  of  the  high-grade  mill  which  has  already  been  described. 

At  the  O'Brien  mill  the  process  is  the  same  mechanically,  but  zinc 
dust  is  replaced  by  aluminum  powder.  It  is  stated  that  the  aluminum 
powder  will  precipitate  three  times  its  weight  of  silver  and  has  a  lesser 


10  DETAILS  OF  CYANIDE  PRACTICE 

fouling  effect  upon  the  solutions.  The  claim  is  also  made  that  the  result- 
ing precipitate  may  be  melted  in  the  ordinary  oil-fired  tilting  furnace 
without  any  flux,  giving  a  bullion  of  high  purity.  The  cost  of  the  alumi- 
num dust,  which  is  32c.  per  lb.,  compared  with  the  cost  of  zinc,  about  7c. 
per  lb.,  does  not  indicate  any  appreciable  economy  in  the  use  of  aluminum 
even  with  its  increased  efficiency,  unless  there  are  some  advantages  not 
clearly  understood.  It  is  probable  that  further  experiments  will  be  made 
on  the  use  of  this  material  so  that  some  definite  facts  may  become  known 
and  comparisons  established  with  the  results  accomplished  with  zinc  dust. 

Vacuum  Filtration. — Vacuum  filters  are  at  present  in  use  in  all  the 
mills,  the  Butters  and  Moore  types  being  the  ones  preferred.  At  the 
Nipissing,  both  the  high-grade  mill  and  the  new  low-grade  mill  are 
equipped  with  Butters  filters  of  the  standard  type.  The  Dominion  mill 
has  a  Moore  filter  as  has  also  the  O'Brien  mill.  In  all  these  mills  a 
high  efficiency  is  claimed  for  the  filters  in  washing  out  dissolved  silver, 
but  from  the  looks  of  the  installations  it  is  rather  difficult  to  understand 
how  high  efficiencies  can  be  claimed  on  any  of  them.  The  great  weight 
of  the  pulp  and  its  granular  condition  make  it  necessary  that  the  filter 
tank  be  equipped  with  air  lifts  in  order  to  keep  the  solids  in  suspension, 
and  a  rather  violent  action  is  necessary  to  do  the  work.  The  filters  cannot 
be  operated  at  all  without  the  air  lifts  as  the  heavy  solids  would-  settle 
immediately  making  operation  impossible.  From  the  nature  of  the  ore 
and  the  condition  of  the  filters  in  the  Cobalt  camp  I  should  think  that 
pressure  filters  would  be  likely  to  give  much  better  results  than  those 
operated  on  the  vacuum  system.  Pressure  systems,  like  the  Merrill, 
would  take  a  definite  quantity  of  pulp  and  make  a  fairly  homogeneous 
cake  at  once,  and  would  tend  toward  a  condition  which  would  facilitate 
proper  washing  of  the  cake.  With  the  vacuum  filters  some  segregation 
must  naturally  take  place,  and  a  filter  cake  which  is  not  homogeneous 
cannot  be  washed  perfectly.  The  pressure  filter  would,  in  my  opinion, 
be  a  decided  improvement  over  vacuum  systems  in  the  Cobalt  camp,  due 
to  the  weight  and  granular  character  of  the  solids  in  the  pulp  treated. 

Cost  of  Operations. — The  cost  of  treatment  in  the  Cobalt  camp  is  not 
obtainable  in  the  majority  of  cases,  due  to  the  fact  that  some  of  the  mills 
are  treating  custom  ores  and  consequently  do  not  care  to  make  public 
that  essential  part  of  the  business.  It  may  be  stated  that  the  treatment 
of  the  mill  or  low-grade  ores  costs  from  $2.50  to  $4.50  per  ton.  This 
seems  to  be  a  rather  wide  variation  of  cost  on  ore  which  is  practically 
identical  and  which  is  treated  on  more  or  less  the  same  scale.  The  varia- 
tion is  wide  indeed,  but,  as  has  already  been  mentioned,  there  are  so  many 
different  ideas  and  so  many  different  methods  in  use  in  the  camp  that 
there  is  bound  to  be  a  variation  in  cost.  Probably  the  cost  for  cyanida- 
tion  is  less  at  the  Buffalo  mill  because  of  the  small  extent  and  simplicity 


THE  COBALT  DISTRICT,  ONTARIO 


11 


of  the  treatment,  most  of  the  charge  for  reduction  being  made  against  the 
concentration  department.  At  the  Buffalo,  the  cost  may  be  said  to  be 
about  the  lower  figure  given,  while  at  the  other  two  mills  costs  varying  at 
or  about  the  higher  figure  may  be  taken  as  representing  the  average.  At 
the  Nipissing  no  costs  at  all  are  divulged,  but  the  report  of  the  company 
for  the  year  1911  shows  that  the  cost  per  ounce  of  silver  at  the  high-grade 
mill  was  1.14c.  At  this  rate,  considering  the  average  ore  treated  as 
assaying  2500  oz.  per  ton,  the  cost  per  ton  of  treatment  would  be  $28.50, 
which  seems  to  be  a  fair  average  figure  for  an  operation  of  this  sort.  It 
seems  likely  that  the  majority  of  this  cost  consists  of  cyanide  consumed  and 
mercury  lost,  the  other  charges  being  of  comparatively  less  importance. 

The  cost  of  material  at  Cobalt  is  about  as  follows:  Cyanide  (KCN 
universally  used)  15c.  per  lb.;  zinc  dust  (90%  metallic  zinc)  6.5  to  7c.  per 
lb.;  pebbles  (imported)  $20  per  ton.  Power  varies  somewhat  in  cost, 
the  original  contracts  being  made  on  a  basis  of  $50  per  horsepower-year, 
but  the  contracts  now  being  made  are  at  the  rate  of  one  cent  per  kilowatt- 
hour,  due.  I  understand,  to  a  change  in  the  policy  of  the  power  companies 
which  serve  the  district.  The  cost  items  in  general  are  not  excessive  and 
work  should  be  done  at  reasonable  cost. 

Buffalo  High-grade  Mill. — The  Buffalo  company  is  erecting  a  small 
mill  to  treat  its  high-grade  mine  product  and  concentrates,  similar  to  the 

MATERIAL  FOR  REFINING  FURNACE  AT  BUFFALO  MINES,  LTD. 


Material 

Dimensions 

Quantity 

Red  brick 

4000 

Firebrick  
Firebrick  

4|X9X2£  in. 
44  and  4X9X3 

1200 
375 

Firebrick  "A"  
Firebrick  "AA" 

4|  and  5^X9X21 
4£  and  5^X9X2^.3^ 

14 
1 

Firebrick  "B"...'  

1\  and  3|X9X4| 

12 

Firebrick  "C"  
Firebrick  "CC" 

26 

2 

Firebrick  "D"  

28 

Firebrick  "DD"  
Firebrick  "E". 

2 
2 

Firebrick  "EE"  

2 

I-beams  
I-beams  . 

3  in.  X  5ft.    6  in. 
4  in.  X6  ft.  6  in. 

6 
12 

Angle  iron  

3  in.  X3  in.  X8  ft.  6  in. 

2 

Angle  iron  
Rods  

3  in.XS  in.X6  ft.  3  in. 
f  in.XS  ft. 

2 

8 

Rods 

\  in.  XlO  ft.  3  in. 

4 

Plates  
Oil  burners,  "Monarch" 

f  in.X4in.X6  in. 

12 

2 

Fire  clay,  cu.  ft 

• 

10 

Sheet  iron  (pan)  '  

3  ft.  8in.X5ft.  ll|in.Xl4 
in.  high 

12 


DETAILS  OF  CYANIDE  PRACTICE 


*t**f*4«k- .,$..> 

/,fZ''  „*        U-.t//   -H 


THE  COBALT  DISTRICT,  ONTARIO  13 

installation  at  the  Nipissing.  The  differences  are  that  the  tube  mill  is 
larger,  5JX22  ft.;  the  agitation  will  be  performed  in  tanks  of  the  Par- 
ral  type;  a  concentrator  with  a  copper  top  will  be  installed  over  which  the 
pulp  will  be  passed  after  amalgamation  in  order  to  recover  any  floured 
quicksilver,  and  filtration  will  be  performed  in  a  large  Perrin  plate-and- 
frame  filter  press  having  a  capacity  of  about  20  tons  of  dry  solids.  Refin- 
ing will  be  performed  as  at  the  Nipissing,  in  a  reverberatory  furnace, 
which  by  courtesy  of  H.  G.  S.  Anderson,  superintendent  of  the  Buffalo 
mill,  I  am  able  to  present  in  detail  in  the  accompanying  drawing.  The 
system  seems  to  be  well  adapted  for  this  class  of  work  and  it  is  not  im- 
possible that  its  use  might  be  profitably  extended  to  other  cyaniding 
installations. 

Future  Development. — The  details  of  cyanidation  of  the  Cobalt 
ores  have  not  yet  been  definitely  settled  and  the  metallurgy  is  in  a  state  of 
evolution  such  that  little  is  really  known.  There  have  been  few  points 
definitely  settled  and  I  expect  to  see  most  of  the  mills  make  radical 
changes  before  settling  down  to  routine  work.  Many  methods  are  in 
use  which  do  not  seem  altogether  suited  to  the  conditions  and  it  is  likely 
that  these  will  be  gradually  changed  as  some  systems  prove  their  super- 
iority. Results  obtained  in  extraction  are  not  yet  reliable,  as  most  of 
them  are  based  on  estimates,  and  the  same  is  true  of  costs,  which  are  so 
varied  and  indefinite  as  to  be  not  suitable  for  comparison.  There  are 
many  skillful  metallurgists  at  work  in  the  district  who  may  be  relied  upon 
to  work  out  the  processes  in  time  and  I  have  no  doubt  that  the  district 
will  reach  a  state  of  settled  and  satisfactory  cyanidation  after  time  has 
been  given  to  allow  the  problems,  which  are  many  and  difficult;  to  be 
worked  out. 


CHAPTER  II 
NIPISSING  HIGH-GRADE  MILL,  COBALT 

The  high-grade  ores  of  the  Cobalt  district  consist  mainly  of  the  arsen- 
ides of  cobalt  and  nickel  in  a  calcite  gangue  with  a  large  quantity  of 
silver  in  the  metallic  state.  A  typical  ore  carries  10%  silver,  9%  cobalt, 
6%  nickel  and  39%  arsenic;  the  rest  is  lime,  silica  and  smaller  amounts  of 
antimony,  iron,  sulphur,  tellurium,  etc. 

Ore  Character. — The  complex  nature  of  the  ore  and  its  high  content 
in  arsenic  make  it  an  undesirable  ore  for  the  ordinary  custom  smelter, 


NIPISSING  HIGH-GRADE  MILL. 


who  is  compelled  to  charge  a  high  price  for  its  reduction.  The  general 
practice  is  to  smelt  in  a  blast  furnace  where  a  large  part  of  the  silver  is 
recovered  at  once  as  base  bullion;  the  resulting  speiss  is  roasted  and 
treated  in  the  wet  way  for  the  recovery  of  the  cobalt,  nickel  and  the  rest 
of  the  silver.  This  process  is  slow;  the  necessary  plant  costs  several 
hundred  thousand  dollars,  and  a  large  working  capital  must  be  available 
to  carry  on  the  business. 

Another  serious  problem  in  shipping  to  a  smelter  was  the  difficulty  in 
obtaining  a  fair  sample  of  the  ore.  This  is  always  a  tender  point  when  a 
mine  is  marketing  ore,  but  when  the  ore  carries  several  thousand  ounces  of 

14 


NIPISSING  HIGH-GRADE  MILL,  COBALT  15 

silver  per  ton  and  the  silver  occurs  in  the  metallic  state,  ranging  in  size 
from  the  smallest  particles  up  to  nuggets  the  size  of  one's  hand,  the  diffi- 
culty of  sampling  is  enormously  increased. 

Great  improvement,  however,  has  been  made  in  this  part  of  the  mar- 
keting by  the  building  of  the  Campbell  &  Deyell  custom  sampling  works 
at  Cobalt,  where  the  ore  is  crushed  in  a  ball  mill  fitted  with  10-  to  20- 
mesh  screens.  All  metallics  coarser  than  this  mesh  remain  in  the  mill  and 
are  subsequently  removed  and  melted  down  to  bullion.  The  pulp 
can  then  be  sampled  with  a  reasonable  degree  of  accuracy. 

The  marketing  expense  is  made  up  of  smelter  deduction,  treatment, 
freight,  assaying,  representation  at  smeltery,  etc.  On  23  carloads  of  ore 
averaging  2560  oz.  per  ton,  the  marketing  expense  was  5.57%  of  the  gross 
value  (with  silver  at  60  c.  per  oz.)  or  $83.55  per  ton.  In  addition  to 
the  above  direct  charge,  there  is  a  loss  of  interest  from  the  date  of  ship- 
ment to  the  date  of  payment,  which  averages  65  days  on  shipments  to 
Canadian  smelters,  and  all  ore  is  settled  for  on  the  commercial  assay, 
which  is  about  1  %  lower  than  the  actual  assay  on  this  grade  of  -ore. 

As  the  largest  producer  in  the  district,  the  Nipissing  Mining  Co.,  Ltd., 
had  been  trying  for  several  years  to  find  a  simple  process  by  which  ite 
high-grade  ores  could  be  treated  at  the  mine  without  going  to  the  largs 
expense  of  building  the  usual  smeltery.  To  this  end  a  great  many  proc- 
esses had  been  investigated  and  one  small  experimental  plant  was 
built;  the  trouble  with  most  of  the  processes  is  the  complication  of  the 
operations  and  the  consequent  tying  up  of  a  large  amount  of  money. 

An  Original  Process. — It  remained  to  Charles  Butters  and  G.  H. 
Clevenger,  finally  to  work  out  a  process  which  promised  to  be  so  simple 
and  effective  that  the  Nipissing  company  decided  to  build  the  necessary 
plant  at  once.  James  Johnston  designed  and  constructed  theplant 
which  went  into  operation  February,  1911,  and  has  run  successfully  ever 
since. 

The  high-grade  ore  from  the  picking  tables  is  delivered  to  the  sampling 
plant  at  the  top  of  the  mill  where  it  is  put  through  a  9  X  15-in.  Blake 
crusher  and  elevated  to  a  steel  receiving  bin.  From  this  it  is  fed  auto- 
matically into  a  No.  3  6-ft.  Krupp  ball  mill  carrying  1000  Ib.  of  steel  balls 
and  fitted  with  20-mesh  screens.  The  metallics  or  silver  nuggets  which 
will  not  pass  the  screen  are  removed  periodically  by  taking  off  a  screen, 
and  are  melted  down  in  the  refinery.  From  the  ball  mill  the  pulp  is 
delivered  by  a  spiral  feed  to  a  Vezin  sampler  and  elevated  to  two  60-ton 
steel  storage  tanks,  from  which  it  is  drawn  as  needed  for  treatment  in  the 
mill. 

The  main  operation  consists  of  amalgamating  the  silver  in  a  5% 
cyanide  solution  while  the  20-mesh  material  is  being  ground  in  a  tube  mill. 
The  mill  used  is  a  Krupp  mill  3  ft.  11  in.  in  diameter  and  19  ft.  8  in.  long, 


16 


DETAILS  OF  CYANIDE  PRACTICE 


fitted  with  silex  liners  and  run  at  37  r.p.m.  The  weight  of  ore  per  charge 
depends  somewhat  on  the  silver  content,  but  with  2500-oz.  ore  the  ordi- 
nary tube-mill  charge  is  6500  Ib.  of  ore;  8500  Ib.  of  mercury;  3800  Ib.  of 
cyanide  solution,  and  six  tons  of  pebbles. 

The  materials  are  charged  through  a  manhole  on  the  top  of  the  mill, 
and  after  the  cover  has  been  replaced  the  mill  is  revolved  for  9J  hr., 
when  99  %  of  the  pulp  will  pass  a  200-mesh  screen.  This  fine  grinding  is 
necessary  to  liberate  the  fine  particles  of  silver  and  permit  of  complete 
amalgamation.  A  screen  analysis  of  the  final  tailing  shows  that  the 
coarser  particles  are  much  richer  than  the  slime;  this  is  also  shown  by  the 
accompanying  screen  tests  on  ore  crushed  through  a  10-mesh  screen. 

It  was  found  advantageous  to  have  a  certain  quantity  of  silver  go  into 
solution  in  the  cyanide,  and  to  this  end  more  air  had  to  be  supplied  to  the 
charge. 

Each  gudgeon  of  the  mill  is  fitted  with  a  stuffing  box  through  which 
passes  a  heavy  cast-iron  pipe,  four  inches  outside  diameter,  with  a  1J- 
in.  hole  through  the  center.  The  casting  is  held  stationary  by  bolts  to 

GRADING  ANALYSIS  OF  NIPISSING  ORE  CRUSHED  THROUGH  10-MESH 


Mesh 

Percentage 
by  weight 

Silver  oz. 
per  ton 

+    20  
+    40 

12.7 
26  2 

6837 
3375 

+    60  
+    80 

11.6 
6  3 

2330 
1954 

+  100  
+  120 

6.3 

2  7 

1654 
1348 

+  150  
+  200 

1.3 

3O 

1182 
1202 

-  200  

29.1 

706 

the  concrete  foundation,  and  the  mill  revolves  about  the  pipe.  Com- 
pressed air  under  25  Ib.  pressure  is  introduced  through  one  of  the  hollow 
castings.  At  the  outlet  end  there  is  a  right-angle  turn  in  the  hollow 
casting  just  inside  the  mill  and  the  upper  end  reaches  to  within  \  in. 
of  the  lining.  The  heavy  cast-iron  elbow,  therefore,  remains  stationary, 
the  inside  leg  stands  vertical,  and  the  upper  end  remains  above  the  level 
of  the  charge  at  all  times,  allowing  the  compressed  air  to  escape  while  the 
mill  is  in  motion.  The  casting  is  heavy  enough  to  withstand  the  batter- 
ing of  the  pebbles  falling  against  it.  This  arrangement  allows  the  mill  to 
be  filled  well  above  the  center  with  a  consequent  decrease  in  the  power 
used,  but  it  is  found  that  the  best  results  are  obtained  by  filling  the 
mill  to  a  point  two  inches  above  the  center. 

Handling  the  Tube-mill  Product. — At  the  end  of  the  grinding  period 
the  three  manhole  covers  are  replaced  by  coarse  screens  and  the  mill  is 


NIPISSING  HIGH-GRADE  MILL,  COBALT  17 

turned  over;  the  charge  falls  into  a  sheet-iron  hopper  which  delivers  it 
into  an  all-iron  settler,  eight  feet  in  diameter,  fitted  with  wooden  shoes. 
The  tube  mill  is  then  washed  out  twice  by  revolving  it  with  a  ton  of  solu- 
tion and  1500  Ib.  of  mercury.  These  washes  are  added  to  the  charge  and 
the  settler  filled  with  solution;  the  charge  is  kept  in  agitation  by  the  mu  - 
ler  while  the  amalgam  is  drawn  off  into  an  iron  clean-up  pan,  and  from 
there  into  canvas  amalgam  filters,  of  which  there  are  24,  each  holding  400 
Ib.  of  amalgam.  The  pulp  is  gradually  run  out  of  the  settler  by  drawing 
the  top  plug,  the  balance  of  the  charge  being  washed  twice  with  solution. 
When  the  flow  of  amalgam  has  ceased,  the  mercury,  as  it  drains  out  of 
the  canvas  filters,  is  pumped  back  to  the  settler  to  wash  out  any  remaining 
amalgam.  The  bottom  plug  is  finally  drawn  and  the  balance  of  the  pulp 
discharged.  It  requires  two  hours  to  dump  the  charge  and  get  the  amal- 
gam into  the  filters. 

It  was  soon  found  that  the  amalgam  must  be  kept  exceedingly  thin, 
otherwise  it  would  stick  in  the  tube  mill  and  cake  under  the  muller  of  the 
settler;  hence  the  mercury  used  is  15  times  the  weight  of  the  silver  in  the 
ore.  After  draining  in  the  sacks,  the  amalgam  still  carries  78%  mercury. 
The  remarkable  part  about  the  whole  process  is  that  97%  to  98%  of 
the  total  silver  in  the  ore  yields  to  amalgamation  in  the  tube  mill.  An 
ore  assaying  2500  oz.  per  ton  is  reduced  to  50  to  75  oz.  per  ton  when  it 
leaves  the  settler. 

The  Role  of  Cyanide. — The  cyanide  treatment  of  the  pulp  which 
follows  is  comparatively  unimportant  as  it  deals  only  with  six  or  seven  tons 
of  50-oz.  ore  daily.  There  are  four  16X7-ft.  wooden  tanks  for  the  col- 
lection and  treatment  of  the  pulp,  and  the  necessary  tanks  for  storage  of 
solution  and  water.  A  charge  for  agitation  is  made  up  of  four  tube-mill 
charges  or  13  tons  of  dry  pulp.  Five  pounds  of  lime  per  dry  ton  of  pulp 
are  added  and  the  charge  is  agitated  for  36  hr. ;  the  tanks  are  fitted  with 
mechanical  agitators,  and  the  pulp  is  circulated  through  a  pump  as  well. 
The  cyanide  strength  is  0.75  per  cent. 

After  settling,  the  solution  is  decanted,  and  the  pulp,  having  a  specific 
gravity  of  2,  is  run  to  a  Butters  filter  of  10  leaves.  The  specific  gravity  of 
the  ore  is  6,  and  to  avoid  the  settling  of  the  pulp  in  the  bottom  of  the  filter 
box  while  the  cake  is  forming,  the  charge  is  kept  in  circulation  by  an 
air  lift  drawing  out  of  the  bottom  of  the  box  and  delivering  at  the  top. 
The  cake  is  washed  2  1/2  hr.  with  weak  solution  and  then  discharged. 
The  arsenides  of  cobalt  and  nickel  go  through  the  process  practically 
unchanged;  the  residue  for  the  first  seven  months  of  this  year  contained 
9%  cobalt  and  4.5%  nickel. 

Analysis  of  Ore. — The  complex  nature  of  this  residue  is  shown  by  the 
following  analysis  by  Johnson  &  Sons  on  some  of  last  year's  product: 
Nickel,  9.72%;  cobalt,  5.85%;  .iron,  2.58%;  antimony,  3.80%;  bis- 


18  DETAILS  OF  CYANIDE  PRACTICE 

muth,  0.09%;  copper,  0.06%;  tellurium,  1.39%;  arsenic,  29.50%;'sul- 
phur,  1.59%;  silica,  11.44%;  lime,  8.63%;  magnesia,  2.91%;  carbonic 
acid,  13.55%;  combined  water,  5.74%;  mercury,  alkali,  oxygen,  gold  and 
silver,  traces  of  zinc,  tin  and  manganese,  3.15%.  Determinations  of 
mercury  consumption  indicated  the  presence  of  mercury  in  the  ore  itself. 
Investigation  along  this  line  showed  that  the  ordinary  high-grade  ore  of 
the  district  usually  carries  from  two  to  five  pounds  of  mercury  per  ton  of 
ore,  depending  upon  the  amount  of  metallics  contained.  Tests  were  then 
made  on  the  metallics  alone;  21  samples  of  metallics  taken  from  various 
Nipissing  veins  and  from  three  other  mines,  showed  mercury  in  every  case ; 
the  result  varied  from  eight  to  95  Ib.  mercury  per  ton  of  metallics  and 
averaged  35  pounds. 

The  presence  of  mercury  in  the  metallics  was  proved  on  a  large  scale 
when  the  refinery  was  built;  the  first  charge  melted  down  in  the  rever- 
beratory  when  the  furnace  and  flues  were  clean,  was  a  ton  of  metallics.  A 
small  bottle  full  of  mercury  was  taken  from  the  condenser  and  globules  of 
mercury  could  be  plainly  seen  through  the  flue  dust. 

Method  of  Precipitation. — There  are  two  zinc  precipitation  boxes, 
one  for  strong  and  one  for  weak  solution.  The  precipitate  from  both 
boxes  goes  to  an  18-in.  Johnson  filter  press  with  12  frames;  the  press 
holds  500  Ib.  of  precipitate  which  assays  15,000  oz.  silver  per  ton.  The 
product  of  the  cyanide  plant  amounts  to  8000  to  10,000  oz.  silver  per 
month. 

The  solution  going  to  the  zinc  box  carries  14  oz.  silver  per  ton,  0.028% 
mercury  and  0.6%  zinc.  By  passing  through  the  box  the  mercury  in 
solution  is  reduced  to  0.015%,  so  it  is  necessary  to  retort  the  precipitate  to 
recover  the  mercury.  As  no  solution  is  thrown  away,  it  has  become  very 
foul;  after  passing  through  the  zinc  box  it  runs  to  a  storage  tank  in  the 
bottom  of  which  a  precipitate  collects.  An  analysis  of  this  precipitate 
follows:  Silver,  0.394%;  mercury,  2.51%;  antimony,  3.30%;  sulphur, 
16.13%;  arsenic,  32.64%;  silica,  5.362%;  zinc,  2.257%;  iron,  5.04%; 
nickel,  9.06%;  cobalt,  7.03%;  lime,  9.24%;  carbon  dioxide,  7.259%; 
manganese,  trace. 

The  sacks  of  amalgam  are  hoisted  into  a  car  with  a  cast-iron  body, 
weighed  and  taken  to  the  refinery  which  adjoins,  but  is  separate  from  the 
mill  building.  There  are  six  14X60-in.  retorts  mounted  in  batteries  of 
two.  They  are  fired  by  oil,  and  the  waste  gases  are  conducted  through 
a  condenser  before  discharging  them;  this  is  to  catch  any  mercury  which 
might  get  into  the  flue  on  account  of  a  cracked  retort. 

The  retorts  are  filled  three-quarters  full  of  amalgam,  fired  for  nine 
hours,  and  allowed  to  cool  for  six  hours.  The  resulting  sponge  and 
mercury  are  weighed  and  compared  with  the  weight  of  amalgam  put  in 
the  retort;  a  cracked  retort  will  sometimes  be  detected  by  the  loss  in 


NIPISSING  HIGH-GRADE  MILL,  COBALT  19 

weight  when  it  has  not  been  noticed  by  the  man  in  charge.  The  sponge 
is  79%  silver;  the  impurities  are  mainly  arsenic,  cobalt,  nickel,  antimony 
and  bismuth. 

The  average  life  of  a  retort  is  34  charges.  As  soon  as  the  slightest 
crack  appears  the  retort  must  be  discarded  at  once.  Occasionally  a 
retort  will  break  on  the  first  firing,  and  retorts  have  been  received  which 
would  not  hold  mercury  when  cold.  Various  grades  of  iron  have  been 
tried  but  the  makers  have  not  succeeded  in  turning  out  a  cast-iron  retort 


RUNNING  THE  SILVER  FROM  FURNACE  INTO  MOLDS. 

which  will  stand  up  to  the  hard  and  continuous  work  required.  A  soft 
gray  iron,  low  in  phosphorus,  gives  the  best  results. 

Recently  a  cast-steel  retort  was  installed;  it  is  still  in  good  condition 
after  having  been  fired  82  times.  A  specially  made  wrought-iron  retort 
with  sides  3/8  in.  thick  is  now  on  order;  this  will  be  slipped  into  a 
cast-iron  cylinder  which  will  take  the  weight  of  the  charge  and  bear  the 
brunt  of  the  firing. 

Melting  of  the  Sponge. — The  sponge  from  the  retorts  is  melted  down 
in  -a  reverberatory  furnace  in  charges  containing  28,000  oz.  of  refined 
silver.  The  hearth  is  stationary  and  is  made  of  brick  contained  in  an 
iron  pan,  5  ft,  X5  ft.  6  in.,  supported  on  rails.  Heat  is  supplied  by  two 


20 


DETAILS  OF  CYANIDE  PRACTICE 


NIPISSING  HIGH-GRADE  MILL,  COBALT  21 

oil  burners;  the  waste  gases  pass  first  through  a  series  of  dust  chambers, 
then  through  two  iron  pipes  3  X  27  ft.  and  to  a  fan  which  exhausts  into  the 
stack.  Sprays  of  water  are  introduced  into  the  last  dust  chamber  and 
into  the  first  iron  pipe,  the  object  being  to  cool  the  gases  and  condense 
any  mercury  that  is  driven  off  in  melting  the  sponge,  or  that  comes  from 
a  leaky  retort. 

As  soon  as  the  charge  in  the  reverberatory  is  melted,  air  under  15  Ib. 
pressure  is  blown  on  the  surface  through  two  iron  pipes  introduced  at  the 
back  of  the  furnace.  The  oxides  of  cobalt,  nickel  and  the  other  impurities 
rapidly  rise  to  the  surface  and  are  scraped  off  through  the  charging  door. 
No  flux  of  any  kind  is  added.  The  air  blown  on  the  surface  of  the  metal 
has  been  heated  by  carrying  the  pipe  through  the  dust  chambers;  this 
materially  shortens  the  operation  which  usually  takes  18  to  20  hr.  After 
the  surface  of  the  metal  has  become  mirror-like  it  must  still  be  blown  sev- 
eral hours  to  expel  the  last  traces  of  impurities. 

Dip  samples  are  taken  and  when  the  bath  is  999  fine  the  air  is  shut  off 
and  the  metal  is  covered  with  charcoal  to  absorb  the  included  oxygen 
and  thus  avoid  sprouting  of  the  bars.  The  furnace  is  tapped  through  the 
side;  the  charge  fills  25  molds  which  are  run  along  under  the  spout  on  a 
car. 

The  first  hearth  was  made  of  common  firebrick;  it  lasted  five  months 
during  which  time  1,860,000  oz.  were  refined.  The  present  hearth 
made  of  magnesite  brick  is  still  in  good  repair  after  nine  months'  use  and 
has  turned  out  to  date  3,236,000  fine  ounces. 

Smelting  the  Precipitate. — For  the  purpose  of  working  up  the  skim- 
ming from  the  melting  furnace,  the  flue  dust  and  the  zinc  precipitate 
from  the  cyanide  plant,  a  20-in.  round  water-jacketed  blast  furnace  was 
installed.  The  jacket  is  hung  from  the  feed  floor  to  allow  removal  of  the 
curb.  The  flue  is  connected  with  the  dust  chambers.  Before  being 
charged  to  the  blast  furnace  the  flue  dust  and  precipitate  are  retorted, 
then  mixed  with  sugar  water,  briquetted  and  dried. 

The  skimming  assays  about  15,000  oz.  silver  per  ton,  the  flue  dust 
700  oz.,  and  the  precipitate  15,000  to  18,000  oz.  The  charge  is  calculated 
according  to  the  material  to  be  put  through;  some  of  the  mixtures  used 
are  as  follows:  Charge  1:  250  Ib.  of  skimming;  300  Ib.  of  slag;  100  Ib.  of 
coke.  Charge  2:  100  Ib.  of  skimming;  100  Ib.  of  flue  dust;  300  Ib.  of 
slag;  40  Ib.  of  iron  ore;  100  Ib.  of  coke.  Charge  3:  10  Ib.  of  flue  dust; 
20  Ib.  of  precipitate;  30  Ib.  of  slag;  25  Ib.  of  iron  ore;  15  Ib.  of  lime- 
stone. These  charges  gave  a  slag  carrying  from  five  to  11  oz.  of  silver 
per  ton. 

The  charge  being  so  rich  in  silver  no  lead  is  necessary.  As  soon  as  the 
crucible  is  half  full  of  bullion  the  metal  is  drawn  off  into  molds.  A  small 
amount  of  speiss  is  also  formed.  The  bullion  runs  800  fine  and  is  melted 


22 


DETAILS  OF  CYANIDE  PRACTICE 


down  in  the  reverberatory  with  the  sponge.     It  is  only  necessary  to  run 
the  blast  furnace  one  day  in  every  10,  to  clean  up  all  the  by-products. 

The  only  product  marketed  is  fine  silver.     It  is  shipped  direct  to 
London  and  sold  at  the  daily  quotation  without  refining  charge.     Ship- 


DryOre  inSack$ 

F~ 

Blake  Crusher 

¥ 
Elevator 


OreBiri 

Ball  Mill  (BO  Mesh) 

^ 
Vezin  Sampler 


Dum 


Butters  Filter 
Tail          Solution  Y/eakSolution 


Amalgam    Mercury 
Tram  Cars 
6-fetorts  (Oil) 
Sponge         Mercury 

-—\  ' — 

Reve r be rcttory  Furnace  (Oil) 

?/fer  Press  '    Skimmings       Fumes        Fine  Si  I  ve  r(999) 

(Retort  and  Briquet)  \ 

.-4- 

Blast -Furnace 

(Pe  tort  and  Briquet) 


FLOW  SHEET  OF  HIGH-GRADE  MILL, 


ments  this  year  have  amounted  to  370,000  oz.  per  month.  The  bullion 
carries  0.0043%  gold,  equal  to  about  $1  per  1200-oz.  bar.  The  total  cost 
of  the  plant  was  $67,757,  which  includes  sampler,  mill  and  refinery. 


CHAPTER  III 
NIPISSING  HIGH-GRADE  MILL,  COBALT— Concluded 

The  works  of  the  Nipissing  Mining  Co.,  Ltd.,  is  situated  on  the  east 
side  of  Cobalt  Lake,  nearly  opposite  the  town  of  Cobalt  in  Ontario.  The 
company  has  had  to  solve  a  metallurgical  problem  that  presented  unusual 
features,  and  has  resulted  in  a  unique  plant  for  milling  the  high-grade 
silver  ore. 

Ore  Character. — The  ore  to  be  treated  contains  a  large  variety  of 
elements,  among  which  may  be  mentioned  about  40%  arsenic,  6%  cobalt, 
6  to  7  %  nickel  and  varying  percentages  of  antimony,  manganese  and  a 
large  amount  of  sulphur 

In  addition  to  the  elements  already  mentioned,  the  ore  is  extremely 
rich  in  silver,  being  indeed  one  which  is  not  usually  considered  in  any 
sense  a  milling  ore.  It  carries  from  2000  to  4000  oz.  per  ton,  an  average  of 
2500  oz.  silver  per  ton  being  usual  over  long  periods  of  time. 

The  suggestion  of  treating  an  ore  of  this  grade,  running  nearly  80  kg. 
per  ton  in  silver,  by  any  milling  process  would  be  treated  warily  by  most 
metallurgists,  the  difficulties  certain  to  be  encountered  being  more  than 
most  would  care  to  face.  The  first  impulse  would  be  to  ship  this  rich 
ore  to  a  smeltery  in  the  confidence  that  a  nearly  total  extraction  might  be 
obtained  at  a  cost  lower  than  could  be  hoped  for  under  milling  conditions. 

After  careful  consideration,  however,  the  Nipissing  Mining  Co.,  Ltd., 
of  which  R.  B.  Watson  is  manager,  came  to  the  conclusion  that  the  high- 
est economy  lay  in  treating  the  ore  on  the  ground,  making  only  one  prod- 
uct— bullion.  The  problems  of  metallurgy  were  placed  in  the  hands 
of  Charles  Butters,  consulting  metallurgist  for  the  company,  assisted  by 
G.  H.  Clevenger  and  James  Johnston,  who  built  the  mill  and  has  since  been 
in  charge  of  it.  The  result  is  the  so-called  high-grade  mill 

Ore  of  this  character,  containing  as  it  does,  pieces  of  rock  of  varying 
grade  from  nothing  up  to  native  silver,  cannot  be  satisfactorily  sampled  in 
a  coarse  state.  At  the  Nipissing  mill  the  sampling  is  done  after  the  ball 
mill  has  crushed  the  entire  material  to  pass  a  20-mesh  screen,  when  it  can 
be  done  with  accuracy 

As  all  the  breaking  and  grinding  up  to  this  point  is  performed  on  the 
dry  ore,  there  is  naturally  some  dust  formed,  and  it  follows  from  the  grade 
of  the  material  that  this  dust  contains  a  valuable  quantity  of  silver.  In 
order  that  this  rich  dust  shall  not  be  lost,  pipes  from  a  small  exhaust  fan 

23 


24  DETAILS  OF  CYANIDE  PRACTICE 

are  led  to  places  where  it  can  best  be  collected,  the  fan  delivering  into  a 
baghouse  from  which  point  it  may  be  returned  to  the  treatment  system. 

The  fine  ore is  taken  to  a  platform  over  a  4X20-ft. 

Krupp  tube  mill.  This  portion  of  the  metallurgy  is  unique  and  somewhat 
astonishing. 

Extraordinary  Amalgamation. — The  tube  mill  has  its  axial  entrances 
sealed  up  except  for  a  small  compressed-air  pipe  which  enters  it  at  one 
end  and  a  corresponding  air  exit  at  the  other  end.  The  mill  has  the  usual 
manholes  in  the  tube,  but  these  are  seldom  used.  Instead  there  are  three 
handholes  with  the  usual  covers  equally  spaced  over  the  length  of  the 
mill  on  the  same  horizontal  line.  Through  these  holes  the  mill  is  charged 
and  discharged,  the  larger  holes  being  necessary  only  when  relining  or 
repairing  inside  the  tube 

Here,  then,  is  a  tube  mill  used  as  an  amalgamating  barrel,  but  under 
conditions  which  were  probably  never  before  sought  for  amalgamating 
purposes.  The  combination  of  an  extremely  high  cyanide  solution,  a 
complex  ore  which  contains  all  sorts  of  elements,  and  mercury  all  ground 
violently  in  a  pebble  mill,  would  seem  to  induce  conditions  quite  the 
reverse  of  satisfactory.  The  natural  expectation  is  that  the  mercury 
would  become  floured  and  that  a  large  loss  of  it  would  be  incurred. 
Particularly  would  this  seem  to  be  the  case  after  the  absorption  of  a 
quantity  of  silver  would  have  resulted  in  the  formation  of  amalgam  with 
a  tendency  toward  granulation,  an  ideal  condition  to  be  thoroughly 
" sickened"  by  the  foreign  elements  in  the  ore. 

It  is  stated  that  this  does  not  occur,  which  must  be  largely  due  to  the 
strong  cyanide  solution  keeping  the  mercury  clean  and  allowing  the  fine 
particles  to  coalesce  without  hindrance  from  a  coating  of  foreign  sub- 
stances. The  heat  formed  in  the  mill  may  also  have  some  effect  in  a 
beneficial  way  by  tending  to  keep  the  mercury  fluid  and  active.  There  is 
a  great  deal  of  heat  generated  in  the  mill,  in  fact  steam  would  be  formed 
were  it  not  for  the  stream  of  compressed  air  which  enters  one  axis  of  the 
mill  and  is  discharged  through  a  standing  pipe  at  the  other.  This  keeps 
down  the  temperature,  though  even  with  this  cooling  assistance  the  tem- 
perature of  the  mixture  after  completion  of  the  agitation  period  is  about 
90  degrees. 

Use  of  Chemicals. — It  ie  noteworthy  that  neither  lime  nor  lead  salts 
are  used  during  this  process,  and  that  after  the  agitation  is  complete,  there 
is  still  sufficient  cyanide  to  carry  on  the  subsequent  operations  without 
further  addition.  The  amount  originally  added  to  the  tube  mill  is 
sufficient  to  produce  a  very  strong  solution,  which  is  capable  of  partially 
outlasting  the  destroying  effect  encountered  within  the  mill. 

The  agitation  is  continued  for  nine  hours,  after  which  time  97%  of 
the  contained  silver  has  been  extracted  from  the  ore.  The  tube  mill 


NIPISSING  HIGH-GRADE  MILL,  COBALT  25 

makes  37  r.p.m.  and,  contrary  to  the  general  practice  of  the  day,  is  lined 
with  silex  brick. 

Power. — The  charge  which  is  put  into  the  mill  fills  it  more  than 
half  full;  there  is  little  space  left  after  putting  in  the  total  charge.  This 
results  in  balancing  the  mill  so  that  during  the  agitation  process  it  requires 
only  about  17  hp.  to  keep  it  in  motion.  When  the  mill  is  emptied 
of  the  charge  and  contains  only  the  pebbles,  it  requires  nearly  40  hp.  to 
move  it 

Cyaniding  the  Pulp. — The  agitating  tanks  are  of  the  flat-bottomed 
type,  having  mechanical  agitating  appliances.  These  consist  of  a  vertical 
shaft,  to  which  arms  are  fixed,  reaching  nearly  to  the  periphery  of  the 
tank.  The  vertical  shaft  is  moved  at  a  rate  of  30  r.p.m.,  by  means  of 
crown  wheel  and  pinion  gearing.  The  high  speed  of  these  agitators 
is  worthy  of  note,  the  necessity  for  it  being  found  in  the  high  specific 
gravity  of  the  ore,  which  is  almost  equal  to  an  ordinary  concentrate. 
Besides,  the  ore  is  so  hard  that  only  a  small  portion  of  it  ever  becomes  a 
true  slime,  but  remains  in  a  granular  form  and  has  that  tendency  to  settle 
characteristic  of  such  material.  The  high  speed  is  necessary  to  keep  the 
solids  in  suspension 

The  striking  feature  of  the  cyanidation  of  the  pulp  is  not  in  the  method 
followed,  which  is  the  usual  one,  but  in  the  fact  that  it  is  carried  out  in 
solutions,  which  must  necessarily  be  in  a  state  which  is  not  usually 
regarded  as  being  efficient  for  cyanidation.  A  consideration  of  what  these 
solutions  must  contain,  coming,  as  they  do,  from  intimate  contact  with 
the  conglomeration  of  elements  found  in  the  tube  mill,  will  indicate  that 
they  must  be  more  or  less  foul  and  in  a  condition  which  is  not  favorable  to 
maximum  efficiency.  This  matter  will  be  referred  to  later 

Precipitating  the  Solution  Content. — The  solutions  are  precipitated 
by  means  of  zinc  shavings  in  a  box  of  the  usual  design,  which  has  eight 
compartments,  each  compartment  having  room  for  about  six  cubic  feet 
of  zinc.  The  shaving  used  is  rather  coarser  than  that  usually  used, 
resembling  a  stout  wire,  the  width  of  the  shaving  being  nearly  equal  to 
the  thickness  of  the  sheet  from  which  it  is  cut 

The  refinery  department  presents  some  unusual  features.  There  are 
six  retorts  of  the  horizontal  tubular  type  set  in  brick,  each  retort  measur- 
ing 14  X  60  in.  These  retorts  are  heated  by  oil  burners,  thus  making  the 
operation  simpler,  cleaner  and  easier  than  when  wood  or  coal  is  used.  The 
oil  burners  are  capable  of  generating  a  degree  of  heat  which  is  entirely 
satisfactory  and  the  operation  is  not  troublesome 

The  precipitate  from  the  cyanide  department  is  mixed  with  a  borax- 
soda  flux  of  the  composition  usual  for  precipitate,  briquetted  and  melted 
in  a  small  blast  furnace,  together  with  the  skimmings  from  the  reverbera- 
tory  furnace  and  other  cleanings  of  value.  This  blast-furnace  system  of 


26  DETAILS  OF  CYANIDE   PRACTICE 

melting  is  not  new,  having  been  practised  before  in  different  places.  It  is 
a  good  system  when  properly  followed,  but  is  not  to  be  promiscuously 
recommended 

From  the  standpoint  of  cyanidation  there  are  several  points  about  this 
metallurgy  which  attract  attention.  The  first  one  is  the  use  of  a  solution 
high  in  cyanide  content  for  the  amalgamation  process.  Of  course,  the 
intention  is  to  use  the  cyanide  entirely  as  an  aid  in  amalgamation,  there 
being  not  the  least  effort  to  put  silver  into  solution  during  this  stage. 
Nevertheless  a  portion  of  the  silver  must  be  dissolved  along  with  a  large 
number  of  other  elements.  The  wonder  is  that  there  is  any  available 
cyanide  left  at  all  after  the  amalgamation  period  is  over. 

Considerations  of  Theory. — A  question  which  is  naturally  suggested  is 
whether  the  mercury  is  dissolved  to  any  extent,  and  if  so,  what  becomes  of 
it.  Some  authorities  claim  that  mercury  in  the  native  state  is  not  dis- 
solved by  cyanide  solutions  while  others  take  the  opposite  view.  I 
am  inclined  to  believe  that  it  depends  a  great  deal  upon  conditions.  A 
volume  of  mercury  in  quiet  contact  with  cyanide  solution  will  probably 
not  dissolve  to  any  great  extent,  but  mercury  in  a  finely  divided  state  and 
amalgamated  with  other  metals  will  be  likely  to  go  into  solution.  Such  a 
finely  divided  and  amalgamated  state  is  certain  to  exist  inside  the  tube 
mill  during  the  amalgamation  period  when  it  is  subjected  to  the  grinding 
action  of  the  pebbles,  and  it  seems  quite  likely  that  a  portion  of  the  mer- 
cury would  be  dissolved.  In  any  case  it  is  well  known  that  some  com- 
pounds of  mercury  are  readily  soluble  in  ordinary  cyanide  solutions  and 
it  is  not  at  all  unlikely  that  some  such  compounds  are  formed  during  the 
amalgamation  period,  so  that  it  seems  reasonable  to  assume  that  there 
would  be  some  mercury  in  the  solution. 

If  this  is  agreed  upon  it  is  easy  to  see  why  lead  salts  are  not  necessary 
in  subsequent  cyanide  treatment,  for  the  mercury  would  decompose  the 
soluble  sulphides  precipitating  HgS.  In  addition,  the  double  cyanide  of 
mercury,  K2Hg(CN)  4,  formed  is  an  active  solvent  of  silver  and  might 
assist  in  recovering  additional  amounts  of  it. 

While  it  has  been  stated  that  no  lime  is  added  to  the  tube  mill  during 
the  amalgamation  period,  the  fact  that  mill  solutions  are  used  makes  it 
necessary  that  lime  in  solution  be  present,  at  least  at  the  initial  period, 
though  it  is  conceivable  that  it  would  be  destroyed  soon  after  agitation 
had  been  started. 

In  addition  to  the  mercury  there  are  elements  in  the  ore,  such  as 
nickel,  cobalt,  manganese,  sulphur,  and  arsenic,  which  would  enter  solu- 
tion. This  presents  an  opportunity  for  all  sorts  of  chemical  reactions 
to  take  place  which  would  be  exceedingly  difficult  to  follow.  The  well 
known  affinity  of  mercury  for  sulphur  would  account  for  its  removal  from 
solution  and  it  seems  likely  that  the  other  elements  would  be  precipitated 


NIPISSING  HIGH-GRADE  MILL,  COBALT  27 

in  some  form  at  some  stage  of  treatment.  It  is  only  reasonable  to  expect 
that  the  solution  of  the  metallic  elements  as  cyanides  and  their  consequent 
transformation  into  other  forms  would  act  as  reducers  to  the  solution, 
robbing  them  of  some  of  the  oxygen  which  would  otherwise  be  available 
and  retarding  the  dissolution  of  silver. 

Character  of  Solutions. — A  peculiarity  of  the  solutions  used  in  this 
treatment  is  that  they  contain  quantities  of  a  brownish-black  precipitate 
floating  in  them.  This  precipitate  is  not  noticeable  until  the  solutions 
enter  the  precipitation  box  where  so  much  of  it  is  liberated  that  it  obscures 
the  zinc.  In  spite  of  this  condition,  which  would  apparently  be  fatal  to 
good  precipitation,  the  results  are  good,  as  shown  by  the  solution  assays 
already  given,  and  the  action  in  the  zinc  box  is  lively  and  normal,  shown 
by  the  evolution  of  gas. 

The  floating  precipitate  is  light  in  weight  and  does  not  remain  on  the 
zinc  to  any  large  extent,  but  does  settle  in  the  sump  tank,  an  analysis  of 
it  showing  that  it  contains  practically  all  the  elements  mentioned  as 
existing  in  the  ore,  together  with  portions  of  zinc  and  mercury. 

The  zinc  in  the  zinc  box  does  not  show  much  effect  of  mercury, 
which  has  the  property  of  amalgamating  with  the  zinc  and  causing  it  to 
break  up  into  a  soft  mass  of  short,  amalgamated  zinc.  There  may  be 
some  slight  indication  of  it,  but  not  enough  to  account  for  the  presence  of 
any  considerable  quantity  of  mercury  in  the  solution 


CHAPTER  IV 


THE  HOLLINGER  MILL,  PORCUPINE 

The  plant  of  the  Hollinger  Gold  Mines,  Ltd.,  is  situated  east  of  Miller 
Lake,  near  the  town  of  Timmins  in  the  Porcupine  district  of  Ontario, 
Canada.  The  mill  occupies  a  site  on  the  side  of  a  hill  in  proximity  to  the 
main  shaft  from  which  the  ore  is  hoisted  from  the  mine.  The  ore  is 
broken  to  a  size  appropriate  for  stamp  milling  at  the  shaft  head  and 
is  then  conveyed  by  means  of  an  inclined  belt  conveyor  to  the  mill  bins, 
where  it  begins  the  process  of  reduction.  The  accompanying  engraving  of 
an  east  view  of  the  mill  shows  the  conveyor  which  carries  the  broken  ore 
to  the  mill  bins. 

Ore  Classification. — The  ore,  in  which  gold  is  the  valuable  constituent, 
is  comparatively  soft  and  is  easily  crushed,  the  crushing  and  grinding 


THE  HOLLINGER  MINE  AND  MILL. 

machinery  having  a  high  rate  of  efficiency,  as  will  be  shown.  The  mill 
run  consists  of  about  two-thirds  sericitic  schist  and  about  one-third  quartz, 
all  the  rock  being  highly  schistose  and  for  this  reason  easily  crushed  and 
ground.  While  a  good  proportion  of  slime  is  formed,  there  is  also  a  large 
proportion  of  fine  sand  in  the  product  which  passes  a  200-mesh  screen. 
Thus  a  product  for  treatment  is  formed  which  is  largely  granular  in 
character  and  which  presents  the  difficulties  usually  encountered  in  a 
pulp  of  this  character.  The  mineral  is  heavy,  the  dry  slime  having  a 
specific  gravity  of  about  2.85.  A  pulp  of  this  character  always  is  trouble- 

28 


THE  HOLLINGER  MILL,  PORCUPINE  29 

some  to  handle  on  account  of  its  tendency  to  settle  at  every  opportunity, 
and  the  machinery  for  handling  it  has  to  be  designed  particularly  with  the 
idea  of  obviating,  as  far  as  possible,  every  chance  for  the  heavy,  granular 
solids  to  settle  and  give  trouble  in  treatment. 

The  ore  from  the  mine  is  first  passed  through  a  No.  7  Kennedy  gy- 
ratory crusher,  after  which  it  goes  through  a  trommel  having  2j-in. 
openings,  the  undersize  going  directly  to  the  20-in.  inclined  belt  conveyor, 
which  carries  it  to  the  mill,  while  the  oversize  is  passed  through  a  20  X 10- 
in.  Allis-Chalmers  Blake  crusher,  which  reduces  all  the  rock  to  2|  in. 
The  crushed  ore  joins  the  undersize  from  the  trommel  on  the  conveyor 
belt. 

The  incline  conveyor  delivers  the  ore  to  a  cross  conveyor  which 
reaches  over  the  top  of  the  mill  bin  and,  by  means  of  a  Bobbins  tripper, 
the  ore  can  be  delivered  at  any  desired  point  in  the  bin.  The  bin  is  of 
wooden  construction  with  flat  bottom  and  has  a  capacity  of  1000  tons  of 
rock. 

Suspended  Challenge  feeders  deliver  the  ore  to  the  stamps,  of  which 
there  are  40  of  1500  Ib.  each,  dropping  in  a  narrow  mortar  of  the  type  used 
for  speed  crushing.  The  height  of  drop  is  6i  in.  and  the  frequency  is 
100  drops  per  minute. 

The  screens  used  in  the  battery  are  of  the  slot  form  of  wire  cloth,  the 
size  used  being  about  six  mesh  in  width  and  about  J  in.  long. 

Experiments  made  on  crushing  capacity  show  that  through  a  14-mesh 
screen  of  this  type  a  capacity  of  eight  tons  per  stamp  per  24  hr.  is  obtained; 
with  6-mesh,  9|  tons  and  with  4-mesh,  12|  tons.  The  screen  normally 
used  at  the  present  time  is  the  6-mesh  and  a  regular  crushing  of  9|  tons, 
possibly  a  little  in  excess  of  this  figure,  is  obtained. 

Crushing  is  done  in  cyanide  solution  of  1J  Ib.  KCN  per  ton,  the 
ratio  being  five  of  solution  to  one  of  ore.  The  mortars  are  arranged  so  that 
the  height  of  discharge  is  two  inches. 

Classifier  and  Tube-mill  Arrangement. — From  the  stamps  the  ore  is 
carried  by  gravity  to  four  Dorr  duplex  classifiers  of  the  Belmont  type. 
This  type  of  machine  is  fitted  with  a  crank  arrangement  with  which  the 
rakes  may  be  lifted  above  the  bed  of  sand  and  the  machine  started. 
This  is  of  great  service  when  a  large  amount  of  sand  has  settled  in  the 
classifier  after  a  shutdown,  the  rakes  being  gradually  lowered  into  the 
settled  sand  and  raking  it  out  by  degrees. ' 

The  sand  from  the  classifiers  is  reground  in  four  Allis-Chalmers  tube 
mills  each  5  X  20  ft.  They  are  lined  with  silex  brick,  the  brick  being  set 
on  edge  in  the  mill,  thus  giving  thickness  which  aids  in  giving  long  life 
of  the  lining.  At  present  no  idea  of  the  life  of  these  linings  can  be  obtained 
because  the  original  linings  are  at  work  and  do  not  show  any  great  indi- 
cations of  wear. 


30 


DETAILS  OF  CYANIDE  PRACTICE 


The  tube  mills  are  fitted  with  special  spiral  scoop  feeders,  22  in.  in 
diameter.  Through  these  feeders  the  daily  addition  of  pebbles  is  also 
put  into  the  mill.  The  pebble  charge  is  about  six  tons  to  each  mill  and 
the  consumption  is  approximately  two  pounds  per  ton  of  ore  ground. 
The  consumption  of  pebbles  was  higher  at  first,  reaching  four  pounds  per 
ton  at  one  time,  but  is  gradually  becoming  less  and  it  is  expected  that  the 
normal  consumption  will  remain  at  about  two  pounds  per  ton. 

The  consistency  of  the  pulp  fed  to  the  tube  mills  varies  somewhat 
from  that  usually  considered  most  efficient  and  is  the  result  of  experiments 
undertaken  to  determine  the  most  efficient  dilution.  The  weight  of  the 
solids  in  this  case  makes  it  most  efficient  to  use  a  thick  pulp,  33  %  moisture 
having  proved  to  give  the  best  results.  The  mills  make  28  r.p.m.,  use 
No.  1  Danish  pebbles  and  grind  90  tons  per  24  hr.  to  a  point  such  that 
90%  passes  a  200-mesh  screen.  The  illustration  shows  the  arrangement 
of  the  stamps,  classifiers  and  tube  mills. 

Driving  the  Stamps  and  Tube  Mills. — At  this  mill  an  original  device, 
by  means  of  which  a  unit  of  10  stamps  and  one  tube  mill  is  driven  from  one 
motor,  is  installed.  The  motor,  of  100  h.p.,  is  connected  with  a  3fi-hi. 
line  shaft  by  means  of  a  Reynolds  silent-chain  drive,  and  this  shaft  passes 


STAMPS  AT  THE  HOLLINGER  MILL. 

to  stamp-mill  line  shaft,  of  2f|  in.,  at  right  angles,  moving  it  by  means  of 
a  miter  gear.  The  motor  shaft  continues  to  the  front  of  the  batteries  and 
is  directly  geared  to  the  tube  mill.  The  advantage  claimed  for  this 
arrangement  is  that  the  full  power  of  the  motor  may  be  utilized  for  start- 
ing the  tube  mill  after  a  shutdown;  the  power  is  sufficient  to  overcome  the 
inertia  of  the  loaded  mill,  the  stamps  being  hung  up  while  this  is  done. 
The  cam  shaft  for  the  10  stamps,  six  inches  in  diameter,  is  in  two  parts 
and  is  driven  by  a  pulley  on  each  end.  The  arrangement  of  the  driving 


THE  HOLLINGER  MILL,  PORCUPINE  31 

motor  and  shafts  is  shown  in  the  accompanying  plan  of  the  mill.  This  ar- 
rangement, while  having  the  advantage  claimed  for  it,  has  also  the  appar- 
ent disadvantage  of  being  subject  to  breakage  due  to  the  crystallization 
of  the  metal  of  the  gears  from  the  constant  vibration  caused  by  the  stamps. 
As  a  matter  of  fact  some  of  these  gears  have  already  broken,  but  it  is 
considered  that  the  advantages  of  the  system  outweigh  the  disadvantages. 

Removing  Gold  by  Concentration. — The  slime  product  from  the  classi- 
fiers is  led  to  a  series  of  wooden  dewatering  boxes,  20  in  number,  each 
6X6X6  ft.,  having  pointed  bottom  and  goose-neck  discharge.  This  de- 
watering  plant  is  really  two  10-compartment  spitzkasten,  and  is  used 
to  obtain  a  pulp  thick  enough  for  efficient  concentration.  The  underflow 
from  the  spitzkasten,  at  a  dilution  of  three  of  solution  to  one  of  solids,  is 
led  to  40  No.  3  Deister  slime  concentrators.  The  concentrators  are  in- 
stalled for  the  sole  purpose  of  removing  from  the  pulp  the  comparatively 
coarse  particles  of  gold  which  would  require  a  long  time  to  dissolve  in  the 
cyanide  solutions  used.  There  is  not  a  great  deal  of  this  gold  that  ever 
appears  on  the  tables,  most  of  it  being  retained  in  the  tube  mill  and 
ground  until  it  is  dissolved  in  the  solution. 

Gold,  even  in  fine  state,  does  not  show  on  the  tables  unless  there  has 
been  an  unusual  quantity  of  extremely  high-grade  ore  milled,  which  is 
exceptional.  The  expectation,  when  the  mill  was  designed,  was  that  there 
would  be  a  large  quantity  of  comparatively  coarse  gold,  which  would 
come  out  of  the  tube  mill  and  would  take  such  a  long  time  to  dissolve  in 
cyanide  solution  that  there  would  be  danger  of  its  being  incompletely  dis- 
solved and  discharged  in  the  tailing.  This  idea  has  proved  to  be  generally 
incorrect,  the  coarser  particles  of  gold  being  ground  up  and  dissolved  to  a 
great  extent  in  the  tube  mill.  The  concentrate  recovered,  however,  is 
high  in  gold  and  is  treated  separately  on  the  ground. 

Amalgamation  of  Concentrate. — The  concentrate  from  the  tables 
drops  directly  into  conduits  in  the  floor,  which  are  equipped  with  spiral 
steel  conveyors  by  means  of  which  it  is  conveyed  to  the  end  of  the  con- 
centrator room,  where  it  is  received  by  a  cross  conveyor  of  the  same  type 
and  delivered  into  the  boot  of  a  belt-and-bucket  elevator.  This  spiral 
conveying  system  would  seem  to  be  a  rather  expensive  way  of  conveying 
concentrate  on  account  of  the  large  amount  of  power  usually  consumed  by 
machinery  of  that  class.  Screw  conveyors  are  usually  not  considered 
altogether  satisfactory  on  that  account. 

The  concentrate  is  elevated  to  a  launder  which  delivers  it  into  four 
wooden,  flat-bottom  bins,  each  4X7 X5  ft.,  where  it  is  stored  and  shoveled 
out  as  required  in  the  amalgamation  treatment. 

There  are  installed  four  Chalmers  &  Williams  standard  Wheeler  pans, 
five  feet  in  diameter,  and  into  each  of  these  is  charged  1.5  tons  of  concen- 
trate, 100  Ib.  of  mercury  and  some  lye  for  keeping  the  mercury  clean,  the 


32 


DETAILS  OF  CYANIDE  PRACTICE 


pan  being  filled  up  with  solution.  The  mullers  of  the  pan  are  let  down  and 
grinding  is  continued  for  one  hour,  after  which  three  hours  are  devoted  to 
amalgamation.  The  pulp  is  then  passed  to  two  8-ft.  settlers,  where 
settling  is  completed  in  four  hours,  the  amalgam  being  drawn  off  and 
cleaned  up  in  a  small  clean-up  pan.  In  this  way  about  97J%  of  the 
gold  contained  in  the  concentrate  is  recovered,  the  amalgam  being  sent  to 
the  refinery,  where  it  is  retorted  and  the  resultant  sponge  added  to  the 
bullion  obtained  from  the  regular  cyanide  treatment. 

As  already  mentioned,  the  original  scheme  of  treatment  included  this 
concentration  and  amalgamation  for  the  purpose  of  taking  care  of  the 
expected  quantity  of  coarse  gold,  but  that  has  not  been  encountered  in 
practice  and  it  is  altogether  probable,  according  to  the  management,  that 


m ; 


| ;  V  }    ITI .  i    .    jj 

«HgiS9  %ffffig$-  -    Waw^MBi*^  I^^B 


EEGRINDING  AND  AMALGAMATING  PANS  AT  THE  HOLLINGER  MILL. 

this  step  in  the  process  can  be  left  out,  the  straight  cyanide  treatment 
being  able  to  recover  as  much  gold  without  it.  Experiments  toward  this 
end  are  being  undertaken  and  a  decision  will  be  made  within  a  few  months 
as  to  the  course  which  will  be  pursued.  The  illustration  shows  the  pres- 
ent installation  of  bins,  pans  and  settlers,  together  with  the  elevator 
which  brings  the  concentrate  from  the  level  of  the  concentrator  room. 

Mechanical  Pulp  Thickening. — The  tailing  from  the  concentrators  is 
joined  by  the  solution  which  was  taken  from  it  by  the  spitzkasten  and 
goes  to  two  Aldrich  triplex  slime  pumps,  each  10X12  in.,  which  deliver 
to  four  Dorr  thickeners.  These  thickener  tanks  are  30X12  ft.  and  the 
scraper  arms  make  f  revolution  per  minute. 

Each  triplex  pump  is  operated  by  a  10-hp.  motor  and  lifts  the  slime  in 
an  8-in.  spiral-riveted  pipe  through  30  ft.  to  deliver  into  the  thickeners. 
The  pulp  from  the  amalgamation  treatment  joins  the  concentrator  tailing 
and  goes  through  the  regular  cyanide  treatment  in  the  mill.  The  Dorr 


THE  HOLLINGER  MILL,  PORCUPINE  33 

thickeners  deliver  an  underflow  containing  48%  moisture  which  is  ele- 
vated in  two  belt-arid-bucket  elevators  with  10-in.  buckets  into  the  treat- 
ment tanks. 

The  pulp  going  into  the  Dorr  thickeners  receives  the  lime  which  is 
required  for  treatment.  The  dry  lime  in  a  fine  state  is  put  into  a  small 
pan  and  an  emulsion  is  made,  which  is  fed  continuously  into  the  pulp  as 
it  enters  the  thickeners.  A  portion  of  the  lime  is  so  coarse  that  it  does  not 
dissolve  or  float  out  of  the  emulsifying  pan,  and  this  portion  is  recovered 
and  put  into  the  mortars  of  the  battery,  where  it  is  soon  ground  up  and 
dissolved.  The  total  quantity  of  lime  added  is  at  the  rate  of  five  pounds 
per  ton  of  dry  ore. 

Agitation  of  Heavy  Solids. — There  are  four  agitating  tanks  each  30  X 15 
ft.  with  flat  bottoms  which  are  equipped  with  Trent  agitators  operated 
by  6-in.  Morris  centrifugal  pumps.  The  pulp  is  treated  continuously 
through  three  of  these  tanks,  the  length  of  time  of  this  passage  being 
about  48  hr.,  experience  having  shown  that  this  is  sufficient  time  for 
treatment;  the  fourth  tank  is  reserved  as  a  spare.  Due  to  the  high  spe- 
cific gravity  of  the  solids  these  agitators  have  been  unsuccessful  in  hand- 
ling the  pulp  in  the  agitation  tanks.  The  pulp  cannot  be  successfully 
agitated  if  this  is  thicker  than  3:1,  the  power  for  moving  the  arms  running 
as  high  as  18  hp.  per  tank. 

At  the  3 : 1  dilution  the  sand  and  granular  slime  settle  in  the  tanks, 
stopping  the  agitator  arms  and  giving  an  endless  amount  of  trouble.  This 
trouble  extends  to  the  filtration  department  where,  on  account  of  having 
to  filter  a  dilute  pulp,  the  operation  cannot  be  performed  in  an  efficient 
manner.  On  account  of  the  difficulty  experienced  with  these  agitators  it 
has  been  decided  to  remove  them  and  install  an  agitator  of  a  different 
type,  which  will  be  described.  With  the  Trent  agitators,  in  addition  to 
the  difficulty  inside  the  tank,  there  is  the  difficulty  experienced  with  the 
use  of  centrifugal  pumps.  With  the  pumps  used  this  consists  principally 
in  the  difficulty  and  time  required  to  take  them  apart  and  examine  the 
interior. 

The  agitators  to  be  installed  are  the  Dorr  type,  which  is  simply  a 
mechanism  with  four  revolving  arms  equipped  with  rakes  exactly  as  in 
the  Dorr  thickener  but  revolving  at  a  higher  speed.  The  slime  is  raked 
down  to  the  center  of  the  tank  and  is  then  lifted  by  means  of  an  air  lift 
situated  in  the  center  of  the  tank,  circulation  being  down  through  the 
tank  and  up  through  the  air  lift  in  the  center.  The  system  has  been 
tried  in  an  experimental  way  at  this  mill  and  there  is  now  under  way  the 
equipment  of  the  spare  agitation  tank,  with  the  agitation  mechanism 
which  will  be  given  a  thorough  trial  on  a  working  scale.  The  arms  will 
make  12  r.p.m.  The  solution  used  in  treatment  is  1.5  Ib.  KCN  per  ton, 
as  is  all  the  solution  used  in  the  mill. 


34 


DETAILS  OF  CYANIDE  PRACTICE 


THE  HOLLINGER  MILL,  PORCUPINE 


35 


36  DETAILS  OF  CYANIDE  PRACTICE 

Vacuum  Filtration  of  Pulp. — The  pulp  from  the  treatment  tanks  is 
sent  to  the  filter  plant  by  gravity.  The  filter  plant  consists  of  a  Moore 
installation  of  four  baskets  of  60  leaves  each,  the  leaves  being  6X8  ft. 
each.  There  are  six  tanks  in  two  units,  each  tank  measuring  28  ft.  4  in. 
wide  and  9  ft.  6  in.  long,  and  having  sides  nine  feet  deep  for  the  vertical 
part,  beside  hopper  bottoms  which  have  7  ft.  6  in.  additional  depth. 
There  are  five  10X7-in.  Buffalo  vacuum  pumps,  one  for  each  basket  and 
one  for  the  acid  washing  of  the  leaves.  One  crane  handles  the  four  bas- 
kets, having  a  capacity  of  35  tons  total  load.  The  crane  has  two  motors, 
one  of  40  hp.  for  the  lift  and  one  of  15  hp.  for  the  lateral  travel.  Some 
trouble  has  been  experienced  with  the  crane,  it  apparently  having  been 
designed  somewhat  light  for  the  work  it  has  to  perform. 

The  cycle  of  operations  is  longer  than  should  be  necessary,  one  hour 
being  allowed  for  loading,  during  which  time  a  cake  If  in.  thick  is 
formed.  One  hour  is  allowed  for  solution  wash  and  five  minutes  for 
the  water  wash.  Transferring  the  basket  takes  about  two  minutes 
and  discharging  the  cake,  which  is  done  in  the  water  washing  tank,  about 
15  min.  The  discharge  is  continuous,  the  tailing  containing  about  25% 
moisture. 

It  is  extremely  difficult  to  get  efficient  washing  under  the  conditions 
obtaining  with  this  filter  because  the  slime  is  so  dilute  and  the  solids  so 
heavy  that  they  will  not  stay  in  suspension,  the  result  being  that  there  is 
a  segregation  in  the  cake,  the  lighter  and  more  impenetrable  slime  being- 
at  the  top  of  the  leaf,  while  further  down  it  is  more  and  more  mixed  with?:' 
granular  product  until  the  bottom  of  the  leaf  contains  a  porous,  granulated^ 
material  which  is  to  all  intents  sand,  and  through  which  the  wash  solution 
will  pass,  leaving  the  upper  part  of  the  leaf,  which  contains  the  true  slime, 
practically  unwashed.     The  filter  tanks  are  supplied  with  air  lifts  to 
assist  in  keeping  the  solids  suspended,  but  they  are  not  altogether  efficient 
and  are  rather  troublesome  to  take  care  of. 

The  major  part  of  this  filter  difficulty  is  traceable  to  the  inefficiency 
of  the  Trent  agitators.  With  a  properly  thickened  slime,  as  has  been 
proven  in  this  case,  less  trouble  will  be  experienced  in  keeping  the  solids 
in  suspension  and  the  homogeneous  cake  thus  formed  can  be  washed  with 
a  97 \%  efficiency.  It  is  expected  that  with  the  installation  of  the  new 
system  of  agitation  a  thick  pulp,  1:1,  will  be  maintained  throughout  the 
agitation  and  filtration  system  with  beneficial  results  to  both. 

The  loading  solution  from  the  filter  plant,  together  with  a  part  of  the 
solution  from  the  Dorr  thickeners,  is  passed  through  two  20-leaf  36-in. 
Merrill  clarifying  filters  and  to  the  pregnant-solution  sumps,  of  which 
there  are  two,  each  22  X  20  ft.  This  solution  is  pumped  out  by  means  of 
a  7X9-in.  Aldrich  triplex  solution  pump,  into  which  zinc  dust  is  fed 
through  a  Merrill  feeder  at  the  rate  of  0.2  Ib.  per  ton  of  solution,  into  two 


THE  HOLLINGER  MILL,  PORCUPINE  37 

20-leaf  52-in.  Merrill  triangular  precipitate  presses,  where  the  precipitate 
is  recovered.  The  regular  addition  of  cyanide  is  made  to  the  gold 
solution  before  precipitation,  the  cyanide  being  ground  up  and  dissolved 
in  the  flow  of  solution.  No  cyanide  is  added  in  any  other  way  in  this 
mill.  The  solution  precipitated  is  about  four  tons  to  every  ton  of  ore 
treated,  the  consumption  of  zinc  being  0.8  Ib.  per  ton  of  ore  milled.  The 
precipitated  solution  is  sampled  for  tonnage  and  assay  purposes  by  a 
tilting-bucket  device. 

The  resulting  precipitate  from  the  cyanide  treatment  is-  fluxed  in  the 
following  proportions:  Precipitate,  100;  borax,  20;  soda,  7;  and  silic  ,  3. 
It  is  melted  in  a  Monarch  tilting  furnace  using  oil  fuel,  in  a  No.  275 
graphite  crucible,  the  bullion  being  remelted  in  a  No.  60  crucible.  The 
resulting  bullion  is  about  760  fine  in  gold.  It  is  contemplated  that  later 
a  lead  stack  will  be  installed  for  the  purpose  of  melting  down  the  precipi- 
tate with  litharge  and  then  cupelling  it  in  an  appropriate  test.  By  this 
means  a  finer  bullion  will  be  produced. 

A  plant  for  sampling  the  ore  going  to  the  mill  will  be  erected  at  some 
time  in  the  near  future.  At  present  every  car  of  ore  is  weighed  and  a 
sample  of  it  taken  so  that  some  idea  of  the  work  done  may  be  obtained. 
The  new  plant  will  contain  Snyder  and  Vezin  samplers  and  a  proportion  of 
the  entire  ore  will  be  cut  out,  reduced  in  small  rolls  and  crushers  and  the 
ensuing  sample  reserved  for  assay. 

The  total  power  required  to  operate  the  mill  is  about  500  h.p.,  elec- 
trical current  being  used  for  all  purposes.  The  mill  is  of  wood  construc- 
tion, well  built,  and  is  covered  with  corrugated  iron.  It  is  sheathed 
inside  with  wood  over  a  layer  of  tarred  paper  and  is  amply  heated  with 
.  steam,  making  a  comfortable  mill  to  work  in  during  the  winter  season. 

Metallurgical  Simplicity. — There  are  no  great  metallurgical  diffi- 
culties, those  that  were  expected  before  operations  were  commenced 
having  failed  to  materialize.  The  ore  is  clean  and  contains  no  rebellious 
elements  and  the  gold  dissolves  easily  in  the  weak  cyanide  solutions  used. 
The  ore  does  not  develop  much  acid  as  is  shown  by  the  small  quantity 
of  lime  used  during  the  treatment.  It  might  be  safely  said  that  the  only 
problems  which  have  been  encountered  are  the  mechanical  ones  already 
mentioned  as  due  to  the  high  specific  gravity  and  granular  character  of  the 
solids  in  the  pulp. 

The  recovery  of  gold  is  thoroughly  satisfactory,  a  total  extraction  of 
93%  being  obtained  even  under  the  present  circumstances  of  mechanical 
imperfection.  Of  the  gold  recovered,  about  85  %  is  dissolved  in  the  grind- 
ing operations,  which  are  intended  to  include  stamping,  classifying  and 
tube  milling,  and  15%  in  the  agitation  department.  The  high  percent- 
age of  gold  dissolved  in  grinding  is  worthy  of  note.  It  is  usually  true 
that  a  large  proportion  of  metals  is  dissolved  in  the  grinding  department, 


38  DETAILS  OF  CYANIDE  PRACTICE 

but  not  much  attention  has  been  given  to  a  study  of  the  fact  and  utiliza- 
tion of  the  opportunity  presented. 

The  dissolving  rate  is  largely  due  to  the  efficiency  of  the  tube  mill  as 
an  agitating  machine,  the  grinding  action  of  pebbles  liberating  and  clean- 
ing the  gold  and  getting  it  in  contact  with  solution  in  a  condition  most 
conducive  to  prompt  dissolution.  For  this  reason  it  is  good  practice  to 
make  the  most  of  the  tube-mill  action  and  do  everything  to  assist  the 
extraction  of  metal  at  this  point.  I  believe  it  is  a  good  thing  to  freshen 
up  the  solution  just  before  going  into  the  tube  mill  either  by  adding  fresh 
cyanide,  part  or  all  of  the  addition  that  is  regularly  made,  or  by  using  a 
solution  which  has  been  freshly  rejuvenated  by  means  of  some  one  of  the 
accepted  methods. 

The  general  run  of  ore  milled  has  a  value  of  about  $20  per  ton,  of  which 
about  98%  is  recovered  in  the  mill.  The  mill  started  operations  in  June 
of  this  year  and  the  greater  part  of  the  elapsed  time  has  been  taken  up  in 
systematizing  the  operation,  making  necessary  minor  changes  and  in 
general  tuning  up  the  mill  so  that  a  normal  basis  of  cost  could  be  obtained. 
The  mill  is  now  running  well  and  the  cost  is  not  excessive,  but  the  manage- 
ment desire  to  establish  the  costs  firmly  before  publishing  any  data. 

The  mill  is  well  built,  compact  and  convenient,  and  reflects  credit  on 
its  constructing  engineer,  A.  G.  Kirby.  The  operations  of  the  Hollinger 
Gold  Mines,  Ltd.,  are  under  the  direction  of  P.  A.  Robbins,  manager,  and 
the  mill  is  superintended  by  Noel  Cunningham. 


CHAPTER  V 
CYANIDING  AT  THE  DOME  MILL 

The  property  of  the  Dome  Mines  Co.,  Ltd.,  is  situated  about  2J 
miles  west  of  the  town  of  South  Porcupine,  Ont  The  property  is  famous 
as  the  first  discovery  in  the  district,  and  has  been  the  subject  of  great  in- 
terest and  discussion  among  mining  men.  It  has  been  developed  for  some 
time,  and  is  now  equipped  with  a  modern  mining  plant,  a  power  plant,  a 
modern  40-stamp  mill  and  cyanide  plant,  started  in  March,  1912. 

Typical  Porcupine  Ore. — The  ore  is  white  quartz  and  schist,  the  quartz 
occurring  as  stringers  and  veins  of  various  width  running  in  different  direc- 
tions through  the  schist  country.  Within  the  payable  zone  the  rock  is  so 
impregnated  with  gold  that  no  attempt  is  made  to  separate  it,  the  whole 
product  being  sent  to  the  mill  as  mined.  The  ore  is  similar  in  character 
to  that  of  the  Hollinger  mine,  but  the  Dome  ore  is  harder,  though  it  does 
not  reach  a  point  where  it  could  be  called  difficult  to  crush.  In  common 
with  the  ores  of  the  camp  and  district  in  general,  it  has  a  high  specific 
gravity,  the  dry  slime  treated  in  the  mill  having  a  specific  gravity  of  about 
2.8,  and  the  resultant  slime  is  largely  granular  in  character.  It  has  been 
determined  that  the  pulp  under  treatment  contains  about  35%  colloid 
slime,  the  rest  of  the  product  being  granular  and  heavy. 

The  mill  is  a  modern  structure  with  steel  frame,  concrete  foundations 
and  floors,  and,  a  feature  which  is  often  forgotten  in  mill  construction, 
excellently  lighted  with  windows  and  skylights,  making  it  easy  and  con- 
venient to  work  in  any  part  of  the  mill  without  artificial  light  in  the  day 
time.  In  view  of  the  prevailing  gloomy  weather  of  the  district,  this  is  a 
distinct  advantage  tending  toward  more  and  better  work  and  has  a  real 
value  in  dollars  and  cents.  The  building  is  heated  by  steam,  a  separate 
boiler  installed  in  the  mill  for  that  purpose  being  of  sufficient  size  to  make 
the  building  comfortable  at  all  times. 

The  ore  going  to  the  mill  comes  from  two  different  places,  two  sets  of 
tracks  entering  the  crusher  house.  The  crusher  house  is  separate  from  the 
mill  and  is  on  the  ground  level.  The  ore,  entering  in  cars  moved  by  a 
hoist  with  an  endless  wire  rope,  is  thrown  first  into  a  No.  7J  Kennedy 
gyratory  crusher.  This  crusher  discharges  to  a  grizzly  with  IJ-in.  open- 
ings. The  oversize  is  put  through  two  No.  3  Kennedy  gyratory  crushers. 
All  the  ore,  after  passing  through  these  crushers,  will  pass  a  IJ-in.  ring. 
This  is  somewhat  smaller  than  is  usually  necessary  for  feeding  stamps,  but 
in  the  case  of  the  Dome  ore  experience  has  proved  that  if  crushing  is  not 

39 


40 


DETAILS  OF  CYANIDE  PRACTICE 


CYANIDING  AT  THE  DOME  MILL 


41 


rather  fine,  slab-shaped  pieces  of  ore  are  likely  to  pass  through  the  crushers 
without  being  sufficiently  reduced.  To  avoid  this  condition,  the  crushers 
are  set  rather  close,  and  the  result  is  that  the  product  is  all  of  a  good  size  to 
go  through  the  feeders  and  into  the  mortars. 

Ore  Delivery  from  Mine  to  Mill. — The  ore  from  the  final  crushers 
falls  upon  an  inclined  belt  conveyor  by  means  of  which  it  is  carried  into 
the  mill  bins.  The  belt  is  20  in.  wide  and  rises  at  an  incline  of  about 
20i°,  the  belt  traveling  at  the  rate  of  350  ft.  per  min.  At  the  point  where 
the  conveyor  enters  the  mill  the  conveyor  way  is  closed  with  an  iron 
door  and  a  heavy  iron  wall  is  interposed  between  it  and  the  mill  build- 
ing, only  a  small  opening  being  left  through  which  the  belt  passes.  This 


DOME  MILL  CRUSHER  HOUSE  AND  INCLINE  IN  FOREGROUND. 


arrangement  is  for  protection  against  fire,  the  crusher  house  and  the 
conveyor  way  not  being  constructed  in  a  way  to  resist  fire  as  the  rest  of 
the  mill  is.  The  half-tone  engraving  shows  the  conveyor  reaching  from 
the  crusher  house  to  the  mill. 

The  inclined  conveyor  delivers  the  ore  to  a  second  conveyor  which 
reaches  across  the  bins.  The  second  conveyor  is  provided  with  a  tripper 
so  that  the  ore  can  be  delivered  into  any  desired  part  of  the  bins.  This 
belt  is  also  20  in.  wide. 

The  mill  bin  is  of  steel  construction  and  has  a  flat  bottom.  It  has  a 
capacity  of  about  1500  tons  of  ore  and  is  placed  back  of  the  stamps.  The 
ore  going  to  the  bins  is  not  weighed,  but  carloads  are  weighed  at  intervals 
and  an  estimation  of  the  quantity  is  made.  A  rough  sample  of  the  coarse 


42  DETAILS  OF  CYANIDE  PRACTICE 

ore  is  taken,  also  one  of  the  pulp  issuing  from  the  battery  at  the  lip  of 
the  mortar,  but  no  absolute  system  has  been  devised  to  record  the 
content  of  the  ore  milled.  According  to  the  general  results  obtained  it 
may  be  said  that  the  ordinary  milling  ore  averages  about  J  oz.  in  gold. 

Crushing  in  Water. — From  the  bins  the  ore  is  fed  into  40  1250-lb. 
stamps  by  means  of  suspended  Challenge  feeders.  The  stamps  drop 
through  6J  in.  100  times  per  minute  and  crush  through  a  rolled  slot  wire 
screen  equal  to  10  mesh.  On  this  ore  and  with  the  screen  mentioned  the 
stamps  have  a  capacity  of  10  tons  per  stamp  per  day,  but  owing  to  the 
limitations  of  the  cyanide  plant  the  actual  crushing  is  a  little  under  that 
figure,  the  40  stamps  crushing  generally  about  380  tons  per  day.  Water 
is  used  in  crushing  at  the  rate  of  about  6  J :  1  of  ore. 

Just  outside  the  batteries  are  the  primary  amalgamating  plates,  one 
for  each  ten  stamps,  54X144  in.,  in  two  sections,  the  grade  being  1J 
in.  per  ft.  These  plates  are  intended  to  recover  coarse  gold  coming  from 
the  batteries,  in  which  there  is  no  provision  for  amalgamation,  but  due  to 
the  scouring  effect  of  the  heavy  rush  of  coarse  pulp  they  are  of  little  use. 
One  of  these  plates  has  already  been  experimentally  discontinued  and  it 
is  likely  that  the  others  will  be  put  out  of  commission. 

From  the  primary  plates  the  pulp  is  passed  through  four  Dorr  duplex 
classifiers  where  the  sand  is  taken  out  and  sent  to  four  Allis-Chalmers 
tube  mills,  each  5X22  ft.,  having  spiral  scoop  feeders  and  reverse-spiral 
discharge,  which  tends  to  prevent  pebbles  issuing  from  the  mill  and  also  is 
convenient  for  the  introduction  of  the  regular  daily  addition  of  pebbles 
which  can  thus  be  fed  into  the  discharge  end  of  the  mill  without  trouble. 
From  the  tube  mills  the  pulp  is  returned  to  the  Dorr  classifiers  by  means 
of  five  Frenier  pumps,  each  8X54  in.,  the  tube  mills  and  classifiers  thus 
being  in  a  closed  circuit.  The  tube  mills  make  32  r.p.m.  which  is  some- 
what excessive,  the  general  speed  of  mills  of  this  size  being  about  28  r.p.m. 
Due  to  the  weight  of  the  pulp  it  may  be  fed  into  the  tube  mills  somewhat 
thicker  than  in  usual  practice. 

From  the  classifier  the  fine  pulp,  or  slime,  goes  over  a  second  series 
of  plates,  called  the  secondary  plates,  where  amalgamation  takes  place. 
These  plates  are  108  X 144  in.  with  a  slope  of  J  in.  per  foot.  On  these 
plates  under  present  conditions  of  operation  about  60  to  65%  of  the  gold 
content  of  the  ore  is  recovered. 

The  whole  of  this  crushing  and  grinding  operation  is  carried  out  in 
water,  the  reason  being  that  a  flow  of  cyanide  solution  is  believed  to  be 
prejudicial  to  good  work  on  the  amalgamating  plates. 

From  the  secondary  plates  the  pulp  is  led  to  three  Dorr  thickeners, 
each  30X10  ft.,  where  the  pulp  is  thickened  as  much  as  possible,  the 
usual  effluent  being  about  1:1.  From  the  thickeners  it  goes  to  the 
boot  of  a  duplex  belt-and-bucket  elevator,  70  ft.  between  centers  and 


CYANIDING  AT  THE  DOME  MILL  43 

carrying  7Xl6-in.  buckets.  This  elevator  delivers  to  the  agitation 
system. 

In  the  boot  of  the  elevator  the  cyanide  necessary  for  treatment  is 
added,  the  material  in  lump  form  being  suspended  in  the  flow  of  pulp 
where  it  dissolves.  The  solution  is  made  to  a  strength  of  one  pound  per 
ton  of  solution,  the  average  consumption  in  total  being  about  0.8  Ib. 
per  ton  of  ore  treated.  The  dilution  ratio  varies  between  1: 1  and  1J:  1. 

The  classification  of  the  pulp  going  to  agitation  is  about  as  follows: 
+  100  mesh,  8%;  +200  mesh,  32%;  -200  mesh,  60%.  The  fineness  of 
grinding  is  controlled  entirely  by  the  comparison  of  cost  and  recovery, 
and  the  most  efficient  condition  for  both  considerations  has  been  found 
to  be  the  condition  above  stated. 

The  water  from  the  pulp  is  returned  to  the  battery  circuit  for  further 
use,  the  circulation  being  maintained  separate.  It  is  of  course  necessary 
to  take  out  as  much  water  as  possible  from  the  pulp  which  is  to  go  to 
cyanidation  treatment,  in  order  that  the  consumption  of  cyanide  shall 
be  as  low  as  possible. 

Pachuca-tank  Agitation. — For  agitation  of  the  pulp  Pachuca  tanks 
are  used,  but  the  design  varies  from  the  usual  standard.  These  tanks 
are  8  ft.  in  diameter  and  40  ft.  deep,  resembling  a  chimney  more  than  a 
tank.  This  form  was  adopted  in  view  of  the  especially  heavy  character 
of  the  slime  to  be  treated,  it  being  considered  that  in  a  wider  tank  there 
would  be  danger  of  settling  of  the  solids.  It  is  stated  that  35  to  40  Ib. 
of  air  pressure  is  sufficient  to  secure  satisfactory  agitation  in  these  tall 
tanks.  They  seem  a  peculiar  form  of  agitator,  but  if  they  are,  as  is 
claimed,  the  most  satisfactory  form,  for  agitating  this  heavy  pulp,  no 
criticism  can  be  made  of  them. 

The  agitation  is  continuous  through  the  Pachuca  tanks,  of  which 
there  are  four,  the  last  tank  being  arranged  so  that  its  overflow  may  take 
out  a  quantity  equal  to  that  which  enters  the  tank,  or  may  be  changed 
so  as  to  take  out  more  or  less,  according  to  the  desirability  of  increasing  or 
reducing  the  amount  of  material  in  the  tank.  The  dilution  of  the  pulp 
during  this  treatment  is  from  1:1  to  1|:1  and  varies  somewhat  in 
accordance  with  the  work  of  the  thickeners.  The  height  of  these  tanks 
results  in  an  advantage  in  that  a  considerable  fall  is  secured  to  the  filters, 
allowing  them  to  be  filled  by  gravity. 

The  pulp  from  the  agitation  system  goes  into  two  Dorr  thickeners, 
each  25  X 10  ft.,  where  the  pulp  is  thickened  for  filtration  and  then  into  a 
mechanical  agitator  tank  where  the  slime  for  filtering  is  stored,  the  slow 
agitation  preventing  the  solids  from  settling  in  the  tank.  The  overflow 
from  these  two  thickeners  is  passed  through  a  Merrill  clarifying  press  and 
is  precipitated. 

Glass  bottoms  are  used  in  many  of  the  launders  at  the  Dome  mill  in 


44  DETAILS  OF  CYANIDE  PRACTICE 

order  to  facilitate  the  flow  of  pulp.  It  has  been  shown  that  with  the  same 
grade  a  launder  having  a  glass  bottom  runs  cleaner  and  easier  and  at  the 
same  time  lasts  longer  than  one  without  it.  The  glass  seems  to  reduce 
friction,  so  that  often  a  launder  which  gives  trouble  when  made  of  wood  or 
iron  will  flow  easily  and  without  trouble  when  glass  bottoms  are  put  in. 
A  sudden  drop  or  fall  of  the  pulp  on  glass  must  be  avoided,  for  in  such 
cases  the  glass  grinds  out  sooner  than  when  iron  is  used.  The  cost  is 
low,  scrap  plate  glass  being  used,  which  may  be  bought  for  about  seven 
cents  per  foot  for  pieces  of  various  length  12  in.  wide. 

Pressure  Filtration. — The  pulp  is  led  from  the  mechanical  agitator 
to  two  Merrill  slime  presses,  standard  type,  having  76  frames  4  inches 
thick.  The  press  is  filled  by  gravity  and  the  slime  being  thick,  there 
is  no  segregation  and  a  homogeneous  cake  is  made.  Once  made,  the  cake 
is  washed  with  barren  solution,  an  amount  equal  to  the  weight  of  the  cake 
being  used.  During  this  wash  there  is  an  appreciable  additional  solution 
of  gold.  After  the  solution  wash  the  cake  is  discharged,  there  being  no 
actual  water  wash  in  the  press,  but  as  it  requires  four  or  five  parts  of 
water  under  80  Ib.  pressure  to  discharge  the  slime,  there  results  a  form  of 
washing,  as  most  of  the  water  is  afterward  recovered  in  the  two  Dorr 
thickeners,  each  30X10  ft.,  through  which  the  sluicing  mixture  is  passed. 

Only  an  equal  amount  of  water  is  discharged  with  the  slime,  the  rest 
being  recovered  from  the  thickeners  and  used  again  to  sluice  out  the 
presses.  The  amount  of  wash  water  discharged  is  about  equal  to  the 
quantity  of  water  taken  into  treatment  from  the  crushing  system.  The 
slime  is  discharged  from  the  thickeners  by  gravity.  This  system  is  well 
adapted  to  the  character  of  the  pulp,  the  heavy  solids  having  no  time  to 
settle  previous  to  filling  the  press,  with  the  result  that  a  cake  which  can 
be  thoroughly  washed  with  solution  is  formed. 

Zinc -dust  Precipitation. — The  Merrill  system  of  zinc-dust  precipita- 
tion is  used,  the  zinc  being  added  to  the  solution  from  the  clarifying  press 
which  passes  to  the  two  pregnant-solution  sumps,  from  where  it  is 
pumped  into  the  precipitation  press,  which  has  ten  2-in.  frames.  The 
zinc  is  added  by  a  spiral-screw  conveyor  which  feeds  the  dust  into  a 
miniature  tube  mill  were  an  emulsion  of  solution  and  zinc  is  formed,  this 
emulsion  going  into  the  stream  of  solution  entering  the  pump. 

The  amount  of  zinc  added  is  approximately  J  Ib.  for  every  ton  of 
solution.  As  there  are  about  two  tons  of  solution  precipitated  for  every 
ton  of  ore  treated,  the  consumption  of  zinc  is  about  i  Ib.,  or  approxi- 
mately two  cents  per  ton  of  ore  milled. 

A  small  amount  of  lime  is  used  in  the  treatment,  the  system  being  the 
formation  of  milk  of  lime  by  placing  the  dry  pulverized  lime  in  a  barrel 
and  adding  a  constant  stream  of  solution,  the  outgoing  solution,  or  milk 
of  lime,  entering  the  treatment  at  the  desired  points.  About  three- 


CYANIDING  AT  THE  DOME  MILL 


45 


fourths  of  the  total  lime  consumption  is  added  to  the  thickeners  which 
prepare  the  pulp  for  the  agitation  treatment,  this  being  to  assist  settle- 
ment and  aid  in  the  production  of  a  thick  pulp;  the  remaining  one-fourth 
is  added  to  the  thick  pulp  going  to  treatment.  The  total  consumption  of 
lime  is  a  little  under  three  pounds  per  ton  of  ore  treated,  as  the  ore  is  not 
extremely  acid,  and  the  lime  is  used  more  to  assist  settlement  than  for  any 
other  purpose. 


Hoi  si- 


No.  7i  Kennedy  Gyratory 
Crushers 


Grizzly  /^  "  Openings 
No.  3  Kennedy  Crushers 


Inclined  Belf  Conveyor 
20"-  Belt 


20"  Cross  Con.^,, 
with  Tripper 

1500  Ton  Flat- bottom 
Steel  Ore  Bin 


Suspended  Challenge 
Feeders 

40- 1250 lt>.  Stamps 

Primary  A  malgamating 
Plates  54"x144" 


Four  Dorr  Duplex 

Classifiers 


Four  5x22  ft.  Tube         -V; 
Mills  32  r.p.m.         "> 


Five  Frenier  Pumps 

S  econdary  Amalgamating 
Plates  108"xl44" 


Three  Dorr  Thickener's 
30x  10  ft. 

Duplex nBucket  Elevator 
7x  16"  Buckets 


Four  continuous  Pachuca 
°  Agitators  8x40  ft 


Zinc  -dust  Emulsified 
Precipitation  Pump 

Precipitation  Press 
Blast  Furnace 


Cupel  Furnace  Oil 

Burner 

Oil-fired  Tilting  Furnace 


FLOW  SHEET  OF  DOME  MILL. 


A  total  of  approximately  95%  of  the  gold  content  of  the  ore  is  re- 
covered, about  65%  of  which  is  recovered  on  the  plates  as  has  been  men- 
tioned, and  the  remaining  quantity  in  the  cyanide  treatment.  The 
percentage  recovery  is  governed  by  the  economic  question,  there  being 
no  difficulty  in  recovering  98  or  99%  of  the  gold  if  it  is  desired  to  do  so. 


46  DETAILS  OF  CYANIDE  PRACTICE 

Experience  has  shown  that  it  is  more  advisable  to  stop  at  the  point  now 
established  than  to  attempt  to  extract  more  of  it.  The  costs  established 
are  proportionately  less  than  if  the  higher  extractions  were  obtained. 

The  power  used  by  the  company  is  generated  in  its  own  power 
house  by  steam,  using  coal  as  fuel,  and  is  higher  than  would  be  the  case 
were  hydro-electric  power  available.  It  is  expected  that  such  power  will 
be  delivered  at  the  plant  within  the  next  few  months,  when  a  corre- 
sponding reduction  of  cost  is  expected.  Detailed  treatment  costs  are  not 
given  at  this  time  as  such  figures  to  be  of  any  service  should  be  repre- 
sentative of  permanent  conditions. 

Smelting  the  Precipitate. — The  precipitate  is  melted  in  a  small  blast 
furnace  with  litharge,  the  resulting  lead  bullion  being  afterward  cupelled 
in  an  appropriate  test.  The  cost  of  this  system  of  refining  seems  to  be 
less  than  the  older  method  of  direct  melting  and  a  practically  fine 
bullion  is  obtained,  with  the  advantages  which  come  with  a  product  of 
that  kind.  The  lead  stack  is  easy  to  operate  and  does  not  entail  the 
severe  labor  attendant  upon  pot  melting,  and  the  cupelling  operation  is 
also  comparatively  clean  and  convenient.  A  comparison  of  the  cost  will 
add  the  deciding  factor,  and  I  expect  to  give  especial  attention  to  this 
matter  in  future  papers  on  the  subject. 

At  the  Dome  mill  the  cupelled  bullion  is  melted  and  made  into  bars  in 
a  small  oil-fired  tilting  furnace.  For  this  purpose  the  tilting  furnace  is 
satisfactory. 

The  cupel  furnace  is  heated  by  means  of  an  oil  burner,  but  tne  operators 
consider  that  it  is  not  as  satisfactory  as  when  wood  fuel  is  used.  The 
cupel  itself  is  made  of  a  mixture  of  ground  limestone  and  Portland  cement 
in  the  porportions  of  about  1  :  3,  which  makes  a  properly  absorbent 
test.  The  saturated  protions  are  melted  down  in  the  blast  furnace  to- 
gether with  scraps,  sweepings,  etc.,  in  a  scavenger  run  which  is  made  at 
intervals. 

Points  for  Discussion. — The  first  point  in  the  Dome  mill  which  is 
open  to  criticism  of  any  kind  is  the  fact  that  no  accurate  weighing  or 
sampling  is  done  on  the  ore  which  enters  the  mill.  Estimations  are 
made,  but  these  can  hardly  be  considered  an  effective  substitute  for 
absolute  methods  and  probably  much  more  satisfaction  would  be  ob- 
tained with  accurate  weighing  and  sampling  systems  such  as  are  being 
installed  in  the  newer  mills.  The  pulp  from  the  Dorr  thickeners  going 
to  the  agitation  treatment  is  estimated  by  measurement  and  specific 
gravity  determinations  which  are  taken  at  regular  intervals  and  the 
operators  believe  that  in  this  way  a  good  estimate  of  the  tonnage  is 
obtained. 

Pachuca  tanks  as  here  used  seem  to  be  a  subject  for  some  discussion, 
but  personally  I  think  there  might  be  some  more  satisfactory  and 


CYANIDING  AT  THE  DOME  MILL  47 

economical  method  of  obtaining  the  same  result.  I  am  not  convinced 
that  Pachuca  tanks  are  most  efficient  agitators,  particularly  where  the 
time  of  contact  is  short  and  where  great  agitation  is  not  necessary. 
The  fall  for  the  filters  could  as  well  be  secured  by  using  tanks  of  different 
form,  even  if  the  elevation  were  made  equal  to  that  now  obtained. 

A  comparison  of  the  design  of  the  Hollinger  and  Dome  mills  presents 
a  good  illustration  of  two  ways  of  doing  the  same  thing.  In  both  mills 
the  original  design  contemplated  the  removal  of  coarse  gold  which 
might  not  be  dissolved  in  the  cyanide  solutions  and  which,  it  was  feared, 
might  be  discharged  undissolved  with  the  residues.  At  the  Hollinger 
the  attempt  was  made  to  extract  this  coarse  gold  on  concentrators,  while 
at  the  Dome  mill  the  same  result  is  sought  through  the  medium  of 
amalgamating  plates.  It  is  probable  that  neither  system  is  necessary. 

At  the  Hollinger  mill  it  has  been  demonstrated  that  this  coarse  gold 
is  more  than  likely  to  be  retained  in  the  tube  mill  as  long  as  it  is  coarse, 
and  being  there  scoured  and  subjected  to  attrition  it  is  generally  reduced 
and  dissolved.  The  concentration  system  is  already  under  suspicion  of 
being  useless  and  may  be  dispensed  with.  At  the  Dome  the  system  of 
amalgamation  on  plates  is  (done  in  water  which  has  to  be  carried  through 
the  whole  grinding  system  and  this  presents  the  serious  defect  of  losing 
the  dissolving  effect  of  the  solution  during  crushing  and  grinding,  and 
particularly  the  highly  efficient  dissolving  action  of  the  tube-milling 
operation. 

I  am  of  the  opinion  that  the  simplification  of  the  mill  through  the 
discarding  of  the  plates  and  grinding  directly  in  cyanide  solutions  would 
obtain  a  result  as  good  or  better  than  the  one  now  in  use  and  perhaps  at 
less  cost.  A  great  advantage  about  the  latter  system  is  that  not  so  much 
water  would  be  necessarily  taken  into  the  circulation.  None  would  be 
taken  in  through  milling  and  what  water  is  required  could  be  taken  into 
circulation  through  giving  the  filter  cake  a  water  wash.  This  would 
reduce  the  loss  in  dissolved  gold  and  also  the  mechanical  loss  of  cyanide, 
which  constitutes  at  present  about  two-thirds  of  the  total  loss. 

The  logical  addition  of  water  to  milling  systems  is  through  the  filter 
cake  where  it  will  do  the  most  good,  and  not  into  the  agitation  circuit 
where  it  merely  dilutes  solutions  and  necessitates  the  addition  of  cyanide 
to  keep  strengths  up  to  normal. 

It  is  still  an  open  question  whether  or  not  economies  could  be  in- 
troduced by  using  a  system  of  partial  leaching,  agitating  only  the  natural 
slime  formed  during  grinding  and  passing  the  fine,  granular  solids  to  a 
leaching  system.  This  question  can  only  be  solved  by  a  series  of  ex- 
periments establishing  the  point  to  which  grinding  must  be  carried  to 
liberate  the  maximum  quantity  of  gold  and  the  desirability  of  either 
leaching  or  agitating  the  fine  sand,  according  to  cost. 


48  DETAILS  OF  CYANIDE  PRACTICE 

The  filtration  system  at  the  Dome  mill,  using  the  Merrill  pressure 
filter,  is  the  best  I  have  seen  in  Canada.  The  character  of  the  pulp 
makes  the  machine  especially  applicable  to  handling  it,  and  filtration  is 
performed  at  a  minimum  expense  and  with  beautiful  simplicity.  The 
pressure  system  ought  to  be  installed  in  all  mills  to  be  built  even  if  it 
does  not  displace  vacuum  plants  already  installed.  With  the  change 
from  the  Dome  practice  of  adding  a  water  wash  to  the  cake,  it  seems  to 
me  that  most  admirable  results  might  be  obtained. 

At  the  Dome  mill  the  metallurgical  difficulties  are  negligible.  The 
problems  are  all  mechanical,  due  to  the  handling  of  the  heavy  pulp  and 
the  devising  of  a  system  of  manipulation  which  will  give  the  highest 
economy. 


DISCUSSION 

Cyaniding  at  the  Dome  Mill 

I  read  with  interest  the  article  by  Mr.  Megraw  in  the  JOURNAL  of 
Nov.  23,  1912,  describing  the  milling  practice  of  the  Dome  Mines  Co., 
at  Porcupine,  Ont.  I  notice  that  the  writer  seems  to  consider  the  ratio 
between  the  width  and  height  (8  : 40)  of  the  Pachuca  agitators  unusual, 
but  in  an  all-sliming  process  such  as  followed  at  the  above-mentioned 
property  it  is  doubtful  if  efficient  agitation  could  be  obtained  with  a 
more  shallow,  wider  tank.  F.  C.  Brown,  in  giving  specifications  for 
Pachuca  tanks  (Min.  and  Sci.  Press,  Sept.  26,  1908),  placed  the  ratio 
between  the  width  and  height  of  standard  tanks  at  1 : 4 \  or  5,  hence  that 
observed  at  the  Dome  plant  does  not  appear  to  be  excessive. 

In  Pachuca  construction  there  seems  to  be  a  general  tendency  to 
build  a  lower,  wider  tank,  either  from  motives  of  economy  in  construction, 
or  from  the  fact  that  in  many  cases  the  tanks  are  simply  remodeled  agita- 
tors of  some  other  type.  While  a  tank  of  this  kind  will  give  fair  results 
when  working  on  a  pure  slime,  in  all-sliming  practice  where  a  large  per- 
centage of  material  consists  of  fine  angular  grains  of  sand,  trouble  is 
bound  to  be  experienced  in  the  close  packing  of  these  particles.  In  fact, 
I  believe  that  the  greater  part  of  the  difficulty  encountered  in  this 
system  of  agitation  is  due  to  the  reason  that  the  proper  ratio  of  width  to 
height  has  been  neglected  in  the  construction  of  the  tanks. 

With  respect  to  the  Dome  plant,  and  taking  into  consideration  the 
all-sliming  method  in  use  there,  I  do  not  see  how  any  other  system  would 
handle  successfully  such  a  heavy  pulp,  containing,  as  it  certainly  must,  a 
large  percentage  of  angular,  fast-settling  particles.  Mechanical  agitation 
under  the  above  conditions  would  entail  an  extremely  high  repair  cost, 
resulting  from  the  strain  on  the  machinery.  The  only  alternative  to  the 


CYANIDING  AT  THE  DOME  MILL  49 

present  treatment,  it  seems  to  me,  would  be  to  divide  the  mill  product 
into  sand  and  slime,  and  treat  each  separately.  Considering  the  matter 
from  this  viewpoint,  and  from  the  fact  that  the  time  spent  in  leaching 
would  be  offset  by  a  lower  power  cost,  I  do  not  see  why  the  older  method 
would  not  be  successful. 

As  regards  amalgamation,  I  am  quite  in  accord  with  Mr.  Megraw;  it 
appears  to  me  unnecessary. 

J.  A.  REID. 
Kingston,  Ont.,  Dec.  1,  1912. 


CHAPTER  VI 
PRACTICE  IN  THE  BLACK  HILLS,  SOUTH  DAKOTA 

The  mineral  district  of  the  Black  Hills,  in  South  Dakota,  has  long; 
occupied  a  prominent  position  in  the  mining  world,  due  to  the  extent  of 
its  deposits  and  their  original  richness.  In  the  early  days  ores  of  high 
grade  were  obtained  and  as  these  gradually  disappeared,  ores  of  lower 
grade  had  to  be  depended  upon  to  keep  up  production.  The  ingenuity 
of  the  metallurgical  world  was  taxed  to  the  utmost  in  devising  methods 
by  means  of  which  profits  might  be  obtained  from  rebellious  ores  contain- 
ing only  small  quantities  of  gold  and  silver.  The  whole  gamut  of 
metallurgical  methods  has  been  played  upon  in  the  effort  to  treat  the  ores 
economically. 

Victory  of  Cyanidation. — Such  processes  as  smelting,  pan  amalga- 
mation, bromination,  chlorination,  plate  amalgamation,  all  had  their  day 
and  were  thoroughly  tried,  but  nothing  made  any  noteworthy  success 
until  cyanidation  was  tried,  proved  available,  and  applied  on  a  large 
scale  to  the  treatment  of  the  ores.  Cyanidation  has  been  the  means  by 
which  enormous  quantities  of  low-grade  ores  have  been  made  economic- 
ally available  and  millions  of  dollars  have  been  taken  from  ores  which 
by  no  other  means  could  be  beneficiated  at  a  profit.  It  is  probable  that 
in  no  other  mining  district  has  cyanidation  had  a  greater  beneficial 
effect. 

The  first  application  of  cyanide  in  the  Black  Hills  was  at  the  Rossiter 
plant  in  1892,  but  great  and  decided  success  was  not  demonstrated  until 
1900,  since  which  time  the  applications  of  the  process  have  increased, 
until  at  present  there  is  no  successful  installation  which  does  not  make 
use  of  the  process  in  some  form. 

The  ores  are  in  the  main  siliceous,  containing  silica  in  the  form  of 
quartz  and  in  proportions  varying  between  75  and  90  %.  The  unaltered 
blue  ores  contain  less  than  the  red  oxidized  mineral.  The  blue  ores 
contain  also  an  average  of  from  6  to  8%  of  pyrites,  fine  and  evenly 
distributed,  though  some  of  them  contain  as  high  as  20%.  In  the  red 
ores  the  pyrite  has  been  oxidized  to  form  the  iron  oxides  which  give  the 
ore  its  color.  Tellurium  has  been  found  in  some  minerals  and  the  exist- 
ence of  tellurides  of  gold  and  silver  is  maintained  by  some  authorities 
Copper  occurs  in  minute  proportions  in  many  of  the  minerals.  At 
some  of  the  larger  mills,  notably  the  Homestake,  copper  is  always  present 

50 


PRACTICE  IN  THE  BLACK  HILLS,  SOUTH  DAKOTA  51 

in  the  precipitate,  but  this  is  believed  to  be  due  principally  to  the  caps 
used  in  detonating  the  charges  of  explosives  and  from  other  similar 
sources,  although  copper  does  exist  in  the  ore.  The  gold  is  in  an  ex- 
tremely fine  state  and  is  rarely  found  jree.  The  ores  are  fairly  hard,  in 
some  instances  extremely  so,  but  there  is  also  much  clayey  material  which 
produces  a  large  amount  of  colloid  slime.  All  grades  of  hardness  are 
found  between  the  extremes  of  clay  and  extremely  hard  and  close- 
grained  rock.  In  most  cases  the  mineral  is  heavy,  the  specific  gravity 
of  the  solids  in  mill  pulp  often  running  as  high  as  3,  while  there  are  many 
graduations  under  that  figure,  all,  however,  being  comparatively  heavy. 

Dry  Crushing  at  Wasp  No.  2. — One  of  the  most  interesting  installa- 
tions in  the  Black  Hills  is  that  of  the  Wasp  No.  2  Mining  Co.,  where  a 
flat  quartzite  deposit  carrying  extremely  small  quantities  of  gold  is  being 
milled  at  a  substantial  profit.  The  ore,  after  passing  the  usual  crusher 
system,  is  broken  through  four  sets  of  rolls,  two  of  which  are  used  for 
roughing  and  two  for  finishing.  This  crushing  is  all  performed  on  the 
dry  ore.  After  passing  the  finishing  roll  the  material  will  all  pass  a 
screen  having  J.in.  openings.  A  large  proportion  of  the  material  is,  of 
course,  much  finer,  varying  between  fine  sand,  there  being  little  slime, 
and  the  maximum  size  of  |-in.  The  crushing  is  carried  no  farther,  the 
ore  being  treated  in  this  condition.  This  finished  product  is  stored  in 
bins  and  is  drawn  out  as  required  to  charge  the  treatment  tanks.  The 
tanks  are  charged  from  these  bins  by  means  of  a  system  of  belt  con- 
veyors which  receive  the  material  at  the  bins  and  deliver  it  at  about  the 
center  of  the  treatment  tanks,  where  it  is  distributed  by  hand.  The 
leaching  tanks  are  six  in  number,  each  32X12  ft.,  and  hold  a  little  over 
400  tons  of  ore. 

Treatment  by  Leaching. — The  cyanide  treatment  of  this  ore  is 
entirely  by  leaching.  The  practice  is  to  add  first  a  bath  of  solution  carry- 
ing five  pounds  KCN  per  ton  in  sufficient  quantity  to  impregnate  the 
charge  thoroughly  and  leave  a  solution  covering  of  about  an  inch  over  it. 
This  bath  remains  in  contact  with  the  charge  for  12  hours,  when  it  is 
drained  off.  The  mixture  of  fine  and  coarse  material  offers  little  re- 
sistance to  the  passage  of  solutions  and  the  leaching  rate  is  exceptionally 
high,  which  facilitates  the  treatment  to  no  small  degree.  It  will  be  seen 
that  this  condition  of  porosity  of  the  charge  also  facilitates  treatment, 
for  the  reason  that  air  for  aeration  of  the  charge  readily  penetrates  the 
entire  charge,  following  the  solution  through  the  mass. 

After  the  strong  solution  has  been  drained  off,  a  weaker  solution  is 
added,  the  strength  being  2J  Ib.  KCN  per  ton,  and  this  treatment  is 
continued  for  48  hours,  there  being  seven  separate  additions  of  the 
solution  during  this  time.  A  subsequent  water  wash  is  given  in  quantity 
only  sufficient  to  displace  the  solution  held  by  the  ore.  The  gold-bearing 


52  DETAILS  OF  CYANIDE  PRACTICE 

solutions  from  this  treatment  are  passed  over  zinc  shavings,  the  resulting 
precipitate  being  dried  and  melted  in  a  crucible  furnace  using  oil  fuel  in 
the  usual  way. 

Tonnage  Estimations. — No  attempt  is  made  to  sample  or  estimate  the 
quantity  of  ore  handled  before  it  enters  treatment,  all  calculations  being 
made  on  the  filled  tanks  before  the  cyanide  solutions  are  added.  In  the 
general  run  of  mills  this  would  be  considered  faulty  practice,  but  in  this 
case  where  the  ore  is  crushed  dry  and  loaded  into  treatment  tanks  without 
any  preliminary  treatment  or  separation  of  any  kind,  it  is  probably  as 
accurate  as  any  way  could  be  and  is  extremely  simple.  The  tanks  can 
be  sampled  satisfactorily  by  means  of  the  pipe  sampler,  taking  a  large 
number  of  tests  in  different  portions  of  the  tank,  and  a  good  estimate 
of  the  weight  can  be  made  from  it  as  well  as  a  satisfactory  sample  for 
determining  the  content  of  the  charge  in  gold  and  silver. 

The  cost  of  discharging  the  tanks  is  low,  approximating  only  about 
five  cents  per  ton.  The  work  is  accomplished  by  seven  men,  four  of 
whom  are  employed  inside  the  tank  shoveling  out  the  charge  through 
bottom  doors,  and  three  employed  in  tramming  out  the  cars,  which  are 
loaded  directly  under  the  discharge  gates.  The  men  inside  the  tank  do 
not  have  to  lift  the  charge  at  all,  the  process  being  simply  to  scrape  the 
material  to  the  discharge  gates.  The  proportion  of  moisture  is  small  and 
the  material  runs  readily  enough  and  gives  no  trouble.  Due  to  this 
facility  the  tanks  may  be  emptied  in  about  seven  hours.  The  tanks 
are  charged  at  a  rate  which  gives  the  mill  a  daily  capacity  of  500  tons. 

Addition  of  Lime. — Lime  at  the  rate  of  six  pounds  per  ton  of  dry  ore 
is  added  at  the  crushers,  and  is  carried  through  the  crushing  system 
with  the  ore,  being  thus  thoroughly  mixed  with  it.  The  treatment 
solutions  carry  about  one  pound  dissolved  CaO,  which  is  sufficient  to 
protect  them  against  any  small  amount  of  acid  which  the  ore  might 
develop. 

The  method  of  adding  lime  seems  to  me  to  be  somewhat  wasteful  in 
this  instance,  as  the  ore  is  not  entirely  reduced  to  a  fine  state  and  it  is 
reasonable  to  suppose  that  the  lime,  at  least  in  part,  will  be  in  coarse 
particles.  The  time  of  treatment  is  not  sufficient  to  dissolve  the  coarser 
pieces  of  lime  entirely  and  it  is  likely  that  an  appreciable  percentage  of  it 
is  discharged  undissolved,  occasioning  a  slight  loss.  I  am  of  the  opinion 
that  some  method  of  adding  the  lime  in  emulsion  in  the  quantity  actually 
required  for  protection  might  result  in  a  slight  reduction  of  the  cost  of 
lime  used.  A  hint  would  be  to  consider  the  method  used  at  the  Home- 
stake  mills  for  adding  lime  to  the  leaching  tanks.  While  this  method 
might  not  be  applicable  in  exactly  the  same  form,  some  simple  variation 
of  it  might  be  productive  of  good  results. 

The  extraction  averages  about  70%  of  the  precious  metals  contained, 


PRACTICE  IN  THE  BLACK  HILLS,  SOUTH  DAKOTA  53 

sometimes  a  little  more  and  occasionally  a  little  less,  depending  on  the 
grade  of  the  ore  handled.  The  cost  of  the  treatment  is  exceptionally 
low.  The  entire  costs  of  the  operations  are:  Mining,  53.48c.  per  ton; 
milling,  66.82c.;  general  expense,  4.35c.  The  principal  items  in  the  mill- 
ing cost  are:  Labor,  21. 3c.  per  ton;  cyanide,  6.3c.;  zinc,  3.3c.;  lime, 
1.2c.;  power,  8.4c.,  and  supplies  and  repairs,  12. 6c.  per  ton  milled. 

Simple  Treatment  at  Wasp  Mill. — It  will  be  seen  that  the  metallurgy 
of  the  Wasp  ore  is  extremely  simple.  The  simplicity  is  probably  due,  in 
a  great  measure,  to  the  fact  that  the  gold  is  contained  in  cleavage  planes 
which  are  fractured  and  opened  by  the  coarse  crushing,  thus  exposing  a 
maximum  quantity  to  the  action  of  cyanide  solutions.  Besides,  the  rock 
itself  is  porous  and  the  solutions  can  readily  enter  and  act  upon  a 
large  portion  of  the  gold,  bringing  it  out  without  entailing  the  expense  of 
fine  crushing. 

A  large  number  of  experiments  have  been  made  on  this  ore  by  com- 
petent metallurgists  in  order  to  determine  whether  an  economical  higher 
extraction  could  be  obtained  by  grinding  finer,  but  the  conclusion  has  been 
that  the  maximum  profit  is  being  obtained  by  the  system  now  followed. 
The  ore  carries  only  about  $2  per  ton  and  will  not  stand  a  higher  operation 
expense.  It  is  likely  that  there  are  few  examples  of  a  similar  nature  in 
existence  where  a  profit  ranging  from  50  to  75c.  per  ton  can  be  obtained 
on  ore  of  this  low  gold  content.  I  have  seen  one  other  example,  a 
Mexican  mine  which  contained  an  ore  having  a  good  gold  content. 
The  ore  was  so  porous  that  pieces  \  in.  in  size  would  readily  surrender 
their  content  to  weak  cyanide  solutions,  but  this  ore  was  in  the  surface 
zone  of  a  vein  and  was  soon  exhausted. 

Wet  Crushing  in  Chilean  Mills. — An  example  of  conditions  differing 
from  those  obtaining  at  the  Wasp  is  found  at  the  mill  of  the  Golden 
Reward  Mining  Co.  at  Dead  wood.  This  company  owns  mines  in 
different  districts  and  the  ores  delivered  at  the  mill  for  treatment  differ 
widely  in  character.  Some  of  them  are  hard,  while  others  are  at  the 
other  extreme  of  the  scale,  being  soft  and  clayey,  many  variations  be- 
tween the  two  being  submitted  for  treatment.  The  ore  is  delivered  to 
the  mill  in  railway  ore  cars  and  is  always  carefully  weighed  before  being 
put  into  the  mill  bins.  The  ore  is  crushed  dry  through  a  series  of  crushers 
and  rolls,  an  automatic  sample  being  taken  during  the  process  which  is 
reduced  in  a  small  crushing  roll,  put  through  several  quartering  samplers, 
and  finally  divided  in  a  small  hand  sampling  machine,  which  delivers  a 
sample  appropriate  for  assaying  purposes.  The  reject  from  the  sampling 
all  goes  into  the  mill  bins  with  the  milling  ore. 

From  the  mill  bins  the  ore  is  taken  to  Chilean  mills  of  the  modern, 
high-speed  type,  making  30  r.p.m.,  and  is  milled  in  cyanide  solution. 
The  feeders  for  these  mills  are  similar  to  the  challenge  feeders  used  for 


54 


DETAILS  OF  CYANIDE  PRACTICE 


feeding  stamps,  but  are  continuously  moved  by  power,  belt  and  pulley, 
and  the  feed  is  arranged  so  that  a  practically  continuous  stream  of  ore  is 
fed  into  the  mills.  The  ore  is  milled  to  pass  a  16-mesh  screen. 

Pulp  Classification. — From  the  Chilean  mills  the  pulp  goes  to  drag 
classifiers  of  the  so-called  Esperanza  type,  which  differ  from  the  Dorr 
machines  in  that  the  series  of  scrapers  is  connected  to  a  link  belt,  which 
moves  continuously  over  sprockets,  large  at  the  slime-discharge  end  of 
the  machine  and  smaller  at  the  sand  end.  These  machines  are  more  or 
less  efficient,  but  the  general  consensus  of  opinion  among  those  who  use 
them  is  that  the  absence  of  the  reciprocal  motion,  which  is  obtained  in  the 
Dorr  machine,  allows  more  slime  to  be  carried  over  with  sand  product. 
The  reciprocal  motion  of  the  latter  seems  to  turn  over  the  sand,  loosen  it 


GOLDEN  REWARD  MILL. 


and  offer  facilities  for  washing  out  the  greater  portion  of  the  slime  mixed 
with  it. 

The  separation  of  slime  and  sand  is  about  half  and  half,  the  sand  being 
delivered  into  leaching  tanks,  each  20X10  ft.,  where  it  is  treated  in  the 
usual  way  by  successive  solution  washes,  followed  by  a  final  water  wash. 
The  slime  is  passed  through  Dorr  thickeners  and  pumped  into  slime- 
storage  tanks,  the  simple  passage  through  the  thickeners  being  sufficient 
to  dissolve  the  economical  maximum  of  the  contained  gold. 

Vacuum  Filtration. — From  the  slime-storage  tanks  the  pulp  is  drawn 
by  gravity  into  the  Moore  filter  at  an  average  dilution  of  about  one  of 
solid  to  1J  of  solution,  this  rate  varying  with  the  exigencies  of  the  oc- 
casion. The  Moore  plant  contains  two  40-leaf  baskets,  each  leaf  measur- 


PRACTICE  IN  THE  BLACK  HILLS,  SOUTH  DAKOTA  55 

ing  6  X  8  ft.  A  1-in.  cake  is  formed  on  the  leaf  in  from  40  to  60  minutes, 
depending  on  the  condition  of  the  leaves.  The  cake  is  washed  with 
solution  and  water  and  discharged  following  the  usual  practice,  the 
application  presenting  no  novelties. 

The  proportion  of  colloid  slime  existing  in  the  pulp  makes  it  par- 
ticularly applicable  to  filtration  by  this  system  and  efficient  results  are 
procured  with  comparative  ease.  The  problem  of  filtration  is  not  a  simple 
one  in  any  case  and  it  cannot  be  said  that  any  of  the  processes  in  use  at 
the  present  time  approximate  perfection  to  any  great  extent,  but  by  apply- 
ing the  machine  and  method  best  adapted  to  any  particular  ore  fairly 
satisfactory  results  may  be  obtained, 

The  ores  treated  at  this  mill  vary  widely  in  grade,  running  from  a 
minimum  of  about  $5  per  ton  to  a  maximum  of  about  $12.  An  average 
extraction  of  slightly  under  80%  is  obtained  at  a  cost  of  less  than  $1.50 
per  ton  milled.  The  lime  used  in  treatment,  the  quantity  varying  with 
the  ore  at  hand,  is  added  at  the  crushers  and  amounts  usually  to  four  to 
six  pounds  per  ton  milled.  The  milling  capacity  is  about  275  tons  per 
day. 

The  practice  of  the  Golden  Reward  mill  is  typical  of  the  modern 
tendency  to  mill  in  cyanide  solutions,  in  this  case  the  strength  being  about 
1 J  Ib.  KCN  per  ton,  and  although  the  mill  is  old  and  not  particularly  well 
adapted  for  securing  low  operation  costs,  the  work  is  nevertheless 
efficiently  done  and  the  costs,  under  the  circumstances,  do  not  seem  to  be 
exorbitant. 

Experimenting  with  Roasting  Process. — The  Golden  Reward  company 
possesses  a  large  quantity  of  ore  which  has  resisted  the  efforts  of  met- 
allurgists to  treat  it  by  any  of  the  straight  milling  processes  and  an  effort 
is  now  being  made  to  render  it  amenable  to  cyanidation  by  giving  it  a 
preliminary  roast.  Experiments  have  shown  that  after  being  roasted 
the  ore  is  amenable  to  cyanidation  and  high  extractions  can  be  obtained 
without  excessive  consumption  of  cyanide  or  other  chemicals.  The 
cost  of  roasting  will  not  be  excessive  as  it  is  proposed  to  utilize  the  sulphur 
content  as  fuel  as  far  as  possible,  thus  reducing  the  consumption  of 
extraneous  fuel. 

A  roasting  furnace  is  now  in  course  of  erection  and  will  be  in  operation 
in  the  near  future.  The  results  obtained  by  this  departure  in  metallur- 
gical practice  will  be  awaited  with  interest. 

Wet  Crushing  through  Rolls. — Another  mill  employing  a  wet  crushing 
process  and  using  cyanide  solutions  throughout  is  that  of  the  Trojan 
Mining  Co.  at  Portland,  probably  one  of  the  most  modern  installa- 
tions in  the  Black  Hills.  This  mill  formerly  was  the  property  of  the 
American  Eagle  company,  but  was  acquired  by  the  Trojan  company  and 
remodeled  to  suit  modern  practice.  The  ore  from  the  mines  is  dumped 


56 


DETAILS  OF  CYANIDE  PRACTICE 


into  bins  at  the  crusher  house  and  from  these  bins  is  drawn  over 
grizzlies,  the  undersize  going  to  a  belt  conveyor,  the  oversize  passing 
through  gyratory  crushers  and  then  joining  the  undersize  on  the  belt 
conveyor.  This  conveyor  has  a  slope  of  about  16°  and  delivers  ore  into 
mill  bins  having  a  capacity  of  about  450  tons.  An  automatic  sampling 
arrangement  was  at  first  installed  to  cut  a  sample  from  the  ore  stream 
falling  into  the  bins,  but  the  arrangement  did  not  give  a  representative 
sample  and  was  dismantled. 

The  ore  is  drawn  from  the  mill  bins  and  fed  by  disk  feeders  into  two 
sets  of  crushing  rolls,  strong  cyanide  solution  being  added  at  this  point. 
These  rolls  were  probably  not  intended  for  wet  crushing,  for  the  housing 
leaks  a  good  deal  and  the  almost  constant  attention  of  an  attendant  is 
necessary  to  keep  the  leaks  stopped  up. 

The  product  of  the  rolls  flows  through  a  launder  to  two  Monadnock 
Chilean  mills,  each  seven  feet  in  diameter.  These  mills  gave  some  trouble 


TROJAN  MILL. 


due  to  slight  mechanical  imperfections,  but  these  have  been  remedied 
so  that  the  mills  now  give  good  results.  The  product  of  the  mills  is 
elevated  to  Dorr  classifiers  where  the  sand  and  slime  are  separated.  The 
sand  is  delivered  to  200-ton  leaching  tanks,  28X8  ft.,  through  automatic 
revolving  arm  distributors.  The  sand  treatment  presents  no  novelties, 
being  the  same  as  that  usually  followed  in  the  district. 

Air  Agitation  of  Slime. — The  slime  from  the  classifiers  is  taken  to 
three  tanks  for  air  agitation,  these  tanks  being  said  to  be  of  the  Pachuca 
type.  They  measure  17  ft.  6  in.  in  diameter  and  16  ft.  in  height,  and  it 
will  be  readily  seen  that  they  differ  a  great  deal  from  the  Pachuca  idea  in 
their  proportions.  A  true  Pachuca  tank  should  have  about  40  ft.  of 
height  for  a  diameter  of  15  ft.  and  the  Trojan  tanks  have  a  greater 
diameter  with  much  less  height.  They  are,  however,  doing  excellent 
work.  The  dilution  of  the  slime  under  treatment  is  about  1  or  1J  :  1,  the 


PRACTICE  IN  THE  BLACK  HILLS,  SOUTH  DAKOTA  57 

object  being  to  maintain  a  fairly  thick  pulp  suitable  for  subsequent 
filtering. 

A  detail  of  these  tanks  which  is  of  interest  is  the  central  air-agitation 
tube,  which  is  not  a  tube  at  all,  but  a  succession  of  cone  sections  set  one 
above  another  through  the  entire  height  of  the  tank.  The  idea  is  that, 
whatever  the  height  of  the  pulp  in  the  tank,  it  can  be  successfully  agitated 
because  the  central  tube  will  discharge  itself  at  almost  any  point. 

The  slime  treatment  is  continuous  through  two  of  the  agitation  tanks, 
the  slime  from  the  first  one  being  transferred  either  to  the  second  or  third 
tank,  as  desired,  by  air  lifts,  and  from  these  secondary  tanks  it  is  drawn 
off  into  the  filter  plant.  The  average  agitation  time  of  the  slime  is  five 
to  six  hours,  no  solution  being  decanted  but  the  entire  pulp  going  to  the 
filter. 

The  solution  fed  into  the  primary  crushing  rolls  with  the  ore  averages 
three  pounds  of  KCN  per  ton  and  is  added  in  the  proportion  of  four  to  six 
tons  of  solution  to  one  of  ore.  The  sand  in  the  leaching  tanks  is  treated 
with  this  same  solution  for  about  three  days,  and  this  treatment  is 
followed  by  treating  two  days  with  weak  solution,  which  has  been 
precipitated,  containing  about  one  pound  of  KCN  per  ton.  A  light 
water  wash  is  given  before  discharging  the  sand,  the  treatment  usually 
extending  over  about  five  days. 

The  filter  is  the  ordinary  Butters  stationary,  semi-gravity  type  which, 
while  more  or  less  satisfactory,  entails  a  high  cost  for  pumping  pulp  and 
solutions.  From  the  filter  the  solution  effluent  from  making  cake  is  sent 
to  the  precipitation  department,  the  weak  barren-solution  wash  and  the 
water  wash  not  being  precipitated. 

Precipitation  is  accomplished  in  the  usual  way,  using  zinc  shavings 
and  the  ordinary  form  of  steel  box.  The  boxes  have  a  total  capacity  of 
384  cu.  ft.  of  zinc.  At  the  cleanup  the  precipitate  is  run  into  an  acid- 
treatment  tank  where  the  sulphuric-acid  process,  customary  in  the  Black 
Hills  plants,  is  carried  out.  The  precipitate  is  collected,  dried,  partly 
roasted,  fluxed  and  melted  in  crucibles  in  a  coke  furnace. 

The  capacity  of  the  Trojan  mill  is  about  175  to  180  tons  per  day,  but 
steps  are  being  taken  to  increase  this  tonnage  materially.  The  extraction 
secured  by  the  combined  slime  and  sand  treatment  is  in  the  neighborhood 
of  75%,  somewhat  more  on  the  slime  than  on  the  sand  product. 

Unusual  Crushing  Practice. — The  practice  of  crushing  with  cyanide 
solution  through  rolls  is  not  usual,  although  it  is  practised  in  a  few  cases 
and  in  one  other  instance  in  this  district,  as  will  be  noted.  It  has  been 
productive  of  good  results,  probably  due  to  the  increased  time  of  contact 
between  the  ore  and  cyanide  solutions,  and  the  agitation  secured  in  the 
rolls  and  Chilean  mills. 

The  Chilean  mills  are  efficient  crushing  machines  and  are  capable  of 


58  DETAILS  OF  CYANIDE  PRACTICE 

handling  large  quantities  of  ore,  but  being  high-speed  machines  they 
naturally  incur  a  maintenance  cost  which  is  high  compared  to  the  results 
obtained  with  slow-speed  mills.  In  this  connection  it  might  be  men- 
tioned that  the  Minnesota  mill,  at  Maitland,  is  equipped  with  slow-speed 
mills  of  the  Lane  type,  and  it  is  claimed  that  these  mills  have  demon- 
strated their  ability  to  crush  an  equal  or  greater  quantity  of  ore  with  less 
expense  both  for  operation  and  maintenance. 

The  Chilean  mill  is  said  to  have  been  introduced  into  Black  Hills 
practice  by  J.  V.  N.  Dorr,  who  first  installed  them  at  the  Lundberg,  Dorr 
&  Wilson  mill  and  afterward  included  them  in  the  design  of  the  Mogul 
mill,  where  they  gave  good  results.  The  Mogul  mill  was  recently 
destroyed  by  fire  and  rebuilding  is  now  under  consideration,  but  the  new 
plant  will  probably  be  built  on  a  site  more  convenient  to  the  mining 
properties  owned  by  the  company. 

Dry  Crushing  of  Hard  Ores. — As  an  example  of  extremely  hard  ores 
found  in  the  Black  Hills  district,  the  material  handled  by  the  Victoria 
mill  in  the  Spearfish  Canon  region  may  be  mentioned.  The  ore  here  is 
extremely  hard  and  the  mill  equipment  has  been  designed  with  this 
characteristic  in  view.  The  ore  is  delivered  into  the  mill  bins  by  a  tram- 
way and  from  these  bins  is  passed  through  a  gyratory  crusher  which 
delivers  a  product  that  will  pass  a  l|-in.  ring.  This  crusher  product  is 
passed  through  a  set  of  rolls  which  delivers  a  product  having  a  maximum 
size  of  f  in. ;  the  crushed  ore  drops  into  a  100-ton  bin. 

From  this  bin  the  ore  passes  a  set  of  fine-crushing  rolls  and  through  a 
trommel,  carrying  6-mesh,  No.  14-wire  screen.  The  oversize  from  the 
trommel  passes  to  a  second  set  of  fine-crushing  rolls,  and  through  another 
trommel  like  that  already  mentioned.  The  oversize  from  this  trommel 
goes  back  to  the  same  rolls  and  the  screened  product  from  both  trommels 
drops  into  a  finished-product  bin  having  a  capacity  of  200  tons. 

The  ore  is  so  hard  that  its  passage  through  the  rolls  is  accompanied 
by  a  great  deal  of  noise  which  sounds  much  like  cannon  shots,  but  the 
rolls  handle  the  material  in  a  satisfactory  manner  without  undue  wear. 

The  finished  product  is  drawn  from  the  bin  and  taken  to  leaching 
tanks  by  means  of  a  belt-conveyor  system.  The  conveyor  deposits  the 
ore  into  the  center  of  the  leaching  tanks,  each  27J  ft.  in  diameter  by  eight 
feet  deep,  with  a  capacity  of  about  200  tons,  and  the  distribution  is 'by 
hand. 

Leaching  is  practised  in  the  ordinary  way  by  first  treating  with  a  3-lb. 
cyanide  solution,  for  72  hr.  During  this  treatment  about  70%  of  the 
gold  content  is  dissolved.  An  additional  10  or  12%  is  obtained  by 
leaching  for  a  further  period  of  48  hr.  with  cyanide  solution  containing 
1J  Ib.  KCN  per  ton,  the  latter  being  finally  displaced  with  a  minimum 
water  wash. 


PRACTICE  IN  THE  BLACK  HILLS,  SOUTH  DAKOTA 


59 


Precipitation  on  Zinc  Shavings. — Precipitation  is  accomplished  in  the 
usual  way,  using  zinc  boxes  of  the  ordinary  type  made  of  sheet  steel. 
There  are  two  six-compartment  boxes,  each  compartment  measuring  two 
by  three  feet.  Each  compartment  is  connected  by  a  pipe  to  a  main  which 
leads  into  the  acid-treatment  tank.  At  the  cleanup  the  zinc  is  thoroughly 
washed  and  the  precipitate,  after  settlement,  is  drawn  through  the  pipes 
directly  to  the  acid  tank  without  further  handling.  The  acid  tank  is 
lead  lined  and  furnished  with  a  hood  by  means  of  which  the  gases  gen- 
erated are  carried  off.  The  acid-treated  product  is  pumped  through  a  filter 
press  and  washed.  This  precipitate,  after  being  partially  dried  and  mixed 
with  the  usual  borax-soda  flux,  is  melted  in  graphite  crucibles.  The 
resulting  bullion  is  a  little  more  than  900  fine,  of  which  50  is  silver. 


LUNDBERG,  DORR  AND  WILSON  MILL. 

The  ore  treated  at  this  mill  varies  in  value  from  $4  to  $7  per  ton.  The 
mill  has  not  been  in  operation  for  a  sufficient  time  to  standardize  the  costs, 
but  it  is  expected  that  these  will  not  average  higher  than  those  obtained 
at  other  similar  mills  in  the  district.  The  mill  is  at  present  treating  about 
200  tons  per  day,  perhaps  the  average  being  a  little  under  that  figure. 

A  Mill  of  Historical  Interest. — At  Terry  is  situated  the  Lundberg, 
Dorr  &  Wilson  wet-crushing  cyanide  mill,  owned  by  a  partnership 
composed  of  John  Lundberg,  J.  V.  N.  Dorr  and  A.  D.  Wilson.  For 
several  reasons  this  plant  is  unique.  It  was  started  in  January,  1904, 
and  was  the  first  mill  in  the  world  to  operate  continuously  and  successfully 
a  leaf-filter  plant.  It  was  in  this  plant  that  the  first  filter  plant  designed 
by  George  Moore  was  installed,  its  difficulties  encountered  and  over- 


60  DETAILS  OF  CYANIDE  PRACTICE 

come;  the  original  plant  is  today  operating  and  doing  good  work.1  In 
this  plant  also  the  well  known  Dorr  classifier  and  the  equally  well  known 
Dorr  thickener  were  developed.  In  addition  this  was  the  first  plant  in 
the  Black  Hills  to  use  the  improved  Chilean  mill  for  crushing  in  cyanide 
solution.  The  plant  has  operated  steadily  since  January,  1904,  except 
during  the  labor  troubles  of  1907  and  1910,  and  it  has  the  best  record  for 
continuous  operation  of  all  the  Black  Hills  cyanide  plants.  The  mill  has 
a  capacity  of  110  tons  per  day  and,  due  to  the  fact  that  it  is  treating 
custom  ores  as  well  as  ores  belonging  to  the  partnership,  few  details  of  the 
practice  have  been  made  public  and  no  information  as  to  costs  has  been 
given  out. 

The  ore  from  the  mines  belonging  to  the  company  is  held  in  three  bins 
having  a  capacity  of  135  tons  and  the  custom  ore  is  dumped  from  mining 
cars  on  the  Chicago  &  Northwestern  R.R.  tracks  in  the  rear  of  the  mill 
into  bins  holding  80  tons,  from  which  point  it  is  trammed  to  the  crushers. 
After  passing  over  grizzlies  with  openings  of  1J  in.  the  ore  passes  through  a 
Gates  gyratory  crusher  and  thence  to  a  12-in.  vertical  belt  elevator  which 
delivers  it  into  a  75-ton  crushed-ore  bin.  As  the  ore  drops  into  this  bin 
an  automatic  sample  is  cut  from  the  stream. 

Wet  Crushing  Rolls. — From  the  crushed-ore  bin  the  material  is  fed 
by  a  cam  feeder  into  a  Carterville  geared  roll  together  with  solution 
carrying  1|  Ib.  KCN  per  ton.  The  solution  is  run  in  sufficient  quantity 
to  sluice  the  product  satisfactorily  through  a  launder  into  the  Chilean 
mill.  This  product,  which  will  average  about  f  in.  in  size,  passes  to  a 
6-ft.  Monadnock  Chilean  mill,  and  is  reduced  so  that  practically  all  of 
it  passes  a  30-mesh  screen.  At  this  point  sufficient  clear  overflow  from 
the  thickeners  and  cones  is  added  to  bring  the  quantity  up  to  three  or  3  J 
tons  of  solution  to  one  of  ore. 

The  ground  product  from  this  mill  is  taken  to  a  standard  Dorr  classi- 
fier, the  sand  product  of  which  passes  to  one  of  four  leaching  tanks, 
each  18  ft.  in  diameter  by  11  ft.  deep,  holding  105  tons  of  sand.  In 
addition  to  the  32  hours  required  to  fill  each  tank,  the  charge  is  given  a 
further  12-hr,  treatment  with  mill  solution.  The  balance  of  the  five-day 
treatment  is  with  barren  solution  carrying  1J  Ib.  cyanide  per  ton,  fol- 
lowed by  a  weak-solution  treatment  and  finally  a  minimum  water  wash. 
The  leaching  rate  starts  at  five  tons  per  hour  and  gradually  diminishes 
toward  the  end  of  treatment  at  which  time  it  is  one  to  1J  tons  per  hour. 

Slime  Treatment. — Slime  from  the  classifier,  which  amounts  to  50 
to  55%  of  the  dry  weight  of  the  ore,  is  divided  between  two  cones,  one 
18  ft.  in  diameter  and  the  other  22  ft.,  and  one  standard  18-ft.  continuous 
Dorr  thickener.  The  two  cones  are  so  placed  as  to  give  a  gravity  flow 

1  This  mill  was  closed  down  late  in  July,  1913,  which  was  subsequent  to  the  date  of 
the  present  article. 


PRACTICE  IN  THE  BLACK  HILLS,  SOUTH. DAKOTA  61 

of  clear  solution  to  the  roll  supply  tank  while  the  overflow  of  the  thick- 
ener goes  to  the  Chilean  mill.  The  thickened  slime  containing  about  55  % 
moisture  is  transferred  by  air  lifts  from  the  thickener  and  the  18-ft.  cone 
to  the  22-ft.  cone.  From  the  latter  it  goes  to  a  small  cone  agitator  and 
into  the  loading  vat  of  the  Moore  filter,  usually  carrying  about  55% 
moisture  and  containing  from  five  to  10%  of  —  200-mesh  material. 

First  Moore  Filter. — This  original  Moore  filter  plant  was  designed  in 
the  beginning  to  handle  30  tons  of  slime  per  day,  but  it  has  been  crowded 
until  it  now  handles  60  tons  daily,  using  two  baskets.  With  minor 
changes  in  the  construction  of  the  plant  and  a  change  in  the  bottom  of  one 
tank  the  installation  is  practically  the  same  today  as  when  it  was  designed 
in  1903. 

The  crane  for  the  transfer  of  the  basket  is  operated  by  water  at  105-lb. 
pressure,  the  water  being  pumped  into  an  accumulator.  It  gives  little 
trouble  and  costs  almost  nothing  for  maintenance.  The  mill  solution 
and  most  of  the  barren-solution  wash  from  the  filter  process  and  sand 
leaching  go  to  the  gold  tanks  and  are  precipitated.  Solutions  assaying 
less  than  $1  per  ton  are  not  precipitated  unless  the  treatment  happens  to 
demand  additional  barren-solution,  or  the  supply  of  high-grade  solution 
is  insufficient  to  keep  the  precipitation  department  busy. 

The  solutions  are  precipitated  by  means  of  zinc  shavings.  In  this 
plant  barrels  have  always  been  used  instead  of  zinc  bozes,  the  plant  having 
a  total  of  30  barrels  in  10  rows  of  three  each.  They  are  cleaned  up  twice 
a  month,  the  rule  being  to  take  all  the  contents  of  the  head  barrel,  part 
from  the  second  barrel  and  still  less  from  the  third  barrel,  sorting  all  the 
coarse  zinc  and  returning  it  to  the  second  barrel.  The  fine  zinc  and 
precipitate  thus  sorted  out  are  placed  directly  in  the  acid  tank.  The 
barrels  are  all  moved  up  one  step,  the  second  barrel  before  cleaning  up 
becoming  the  first  and  the  first  barrel  moved  down  to  the  foot.  The 
second  barrel,  now  the  head  of  the  series,  is  repacked,  the  next  partially 
repacked;  all  of  the  fine  material  taken  out  is  put  in  the  acid  tank.  As 
the  cleanup  usually  consists  of  about  100  Ib.  of  dry  product,  all  the  figures 
which  follow  apply  to  a  cleanup  of  that  size. 

To  the  product  in  the  acid  tank  415  Ib.  of  commercial  sulphuric  acid 
are  added  and  allowed  to  remain  in  contact  for  six  to  seven  hours,  or 
until  chemical  action  has  nearly  ceased.  Water  in  equal  volume  is  then 
added  and  the  contents  are  heated  with  live  steam  and  allowed  to  stand 
10  or  12  hr.,  usually  over  night.  The  following  morning  the  solution  is 
siphoned  into  a  settling  tank  where  any  flocculent  matter  is  settled.  An- 
other water  wash  is  added  and  live  steam  again  introduced  to  heat  the 
charge  thoroughly.  The  charge  is  then  allowed  to  stand  about  an  hour. 
The  clear  solution  is  drawn  off  into  a  vacuum  tank  and  filtered.  When 
the  acid  tank  is  nearly  empty  the  contents  are  energetically  stirred  to 


62  DETAILS  OF  CYANIDE  PRACTICE 

bring  all  of  the  product  into  suspension  and  the  charge  is  transferred  to  a 
vacuum  tank  and  filtered. 

When  the  moisture  contained  is  brought  down  to  4  or  5  %  the  product 
is  taken  from  the  tank,  broken  so  that  no  piece  is  larger  than  a  1-in.  cube 
and  placed  in  flat,  iron  roasting  pans.  These  pans  are  placed  in  a  soft- 
coal  furnace  and  the  fire  is  started.  The  furnace  has  a  total  capacity  of 
four  18X3-in.  pans  in  two  layers;  1J  to  two  hours'  time  is  sufficient  to 
drive  off  nearly  all  the  zinc,  and  the  resulting  bullion  is  comparatively 
clean.  •  The  product  in  the  pans  is  not  touched  or  stirred  during  the  roast- 
ing and  experiments  have  proved  that  mechanical  losses  are  thereby 
eliminated.  The  size  to  which  the  product  is  broken  is  small  enough  to 
insure  a  good  clean  roast. 

The  roasted  precipitate  is  placed  in  a  No.  100  graphite  crucible  in  a 
coke  furnace  together  with  five  to  seven  pounds  of  flux  composed  of  f 
borax  glass  and  -J  sodium  carbonate.  Melting  is  usually  completed  in 
about  1|  hr.,  using  80  to  100  Ib.  of  coke.  The  bar  is  poured  into  a  bullion 
mold  as  it  is  not  necessary  to  remelt.  The  resulting  bullion  is  950  to 
970  fine.  Bars  have  been  made  as  high  as  984  fine,  but  the  average  is  as 
stated  above. 

Although  the  mill  was  built  seven  years  ago  it  is  kept  in  the  best  repair 
and  is  still  uptodate  in  most  of  its  features.  The  machinery  is  driven  by 
electricity  throughout,  using  alternating  current  at  440  volts;  a  total  of 
104  hp.  is  used. 

Other  Mills  in  the  District. — Among  the  mills  now  operating  in  the 
Black  Hills  district  may  be  mentioned  the  new  Reliance,  which  has  just 
been  remodeled  and  has  started  operation  along  lines  unusual  in  the  camp, 
treating  slime  by  a  continuous  decantation  process  and  making  use  of 
Dorr  thickeners  for  the  purpose.  This  mill  has  also  installed  a  Portland 
continuous  filter  for  filtering  the  slime  tailing.  The  mill  has  been  in 
operation  only  a  short  time,  remodeling  is  not  yet  complete  and  no 
information  is  available  for  publication. 

The  Bismark  Mining  Co.  is  building  a  mill  near  the  Wasp  No.  2  in 
which  the  system  of  treatment  will  be  identical  with  that  of  the  latter. 
The  mill  is  approaching  completion  and  should  be  in  operation  in  the 
near  future. 

In  this  paper  no  mention  has  been  made  of  the  metallurgy  of  the 
Homestake  installations.  This  is  a  matter  which  is  so  extensive  that  it 
should  be  treated  alone  and  I  hope  in  a  later  paper  to  discuss  that  practice. 
The  Homestake  is  treating  ores  on  a  scale  which  is  not  equaled  at  the 
present  time  and  has  attained  an  astonishingly  low  cost  for  mining  and 
milling.  An  ore  of  low  grade  is  being  treated  and  the  methods,  original 
in  many  instances  as  they  are  efficient,  are  a  lasting  credit  to  the  energy 
of  the  technical  men  who  are  responsible  for  their  devising. 


PRACTICE  IN  THE  BLACK  HILLS,  SOUTH  DAKOTA  63 

There  are  many  mills  in  the  Black  Hills  which  are  not  now  operating, 
but  it  would  serve  no  useful  purpose  to  speak  of  these  at  the  present  time 
in  spite  of  the  fact  that  some  of  them  may  resume  operations  in  the  near 
future. 

Conclusions. — It  is  with  no  little  hesitation  that  one  ventures  to  call 
attention  to  details  in  the  practice  of  this  district  which  seem  capable  of 
improvement,  in  view  of  the  fact  that  capable  operators  have  studied 
the  problems  and  undoubtedly  are  aware  of  the  solutions  of  them  even  if 
they  are  not  put  into  practice  at  the  present  time.  A  little  criticism, 
however,  may  be  helpful  from  a  constructive  point  of  view  and  the  few 
suggestions  that  I  venture  to  make  are  offered  in  a  friendly  spirit,  with  a 


NEW  RELIANCE  MILL. 

desire  only  to  add  a  little  to  the  present  practice.  It  is  possible  that  a 
perspective  view  may  be  appreciated  by  those  whose  vision  has  been 
limited  by  long  contact  and  short  focus. 

The  first  impression  is  that  in  few  of  the  operating  plants  is  there  any 
efficient  means  of  estimating  with  accuracy  the  tonnage  treated.  The 
importance  of  the  point  will  be  readily  appreciated  by  those  familiar  with 
the  problem,  so  that  it  needs  no  discussion.  It  will  be  sufficient  to  say 
that  mill  estimations  are  not  sufficiently  accurate  upon  which  to  base 
comparative  calculations  except  in  such  extraordinary  cases  as  has  al- 
ready been  mentioned  in  the  Wasp  No.  2  mill.  A  system  of  weighing  the 
mill  ore,  while  expensive  to  install,  will  repay,  in  information  acquired  and 
in  satisfaction,  any  financial  outlay  which  may  be  necessary. 

The  same  is  true  of  sampling.  Sampling  plants  are  expensive  to  install 
but  are  productive  of  much  good.  In  some  mills  in  the  district  samples  of 
mill  run  are  taken  by  means  of  a  large  elevator  bucket  fixed  to  a  chain 
belt  which  periodically  crosses  the  stream  of  ore  falling  into  the  mill 
bins.  This  device  is  more  than  likely  to  give  erratic  results.  The 
area  of  the  bucket  opening  is  too  small,  and  large  pieces  of  ore  strike  the 


64 


DETAILS  OF  CYANIDE  PRACTICE 


PRACTICE  IN  THE  BLACK  HILLS,  SOUTH  DAKOTA 


65 


66  DETAILS  OF  CYANIDE  PRACTICE 

edges  of  the  bucket  and  bounce  off,  leaving  a  sample  which  is  not  repre- 
sentative. Often  the  capacity  of  the  bucket  is  not  sufficient  to  hold  the 
entire  quantity  of  ore  which  belongs  to  it  and  it  piles  up  and  overflows, 
again  resulting  in  an  erratic  sample.  I  believe  it  is  recognized  that  to 
secure  a  proper  sample  it  is  necessary  to  take  the  whole  stream  of  ore  at 
regular  intervals,  the  oftener  the  better,  and  to  reduce  the  size  of  the 
sample  by  successive  operations  of  the  same  kind.  A  part  of  the  stream 
for  all  or  part  of  the  time  is  not  sufficient  and  leads  to  a  sample  which  is 
not  accurate,  and  this,  in  the  opinion  of  those  who  have  most  carefully 
studied  the  subject,  is  worse  than  no  sample  at  all. 

Crushing  Hard  Ore  with  Rolls. — The  practice  of  crushing  through 
rolls  in  this  district  should  be  an  object  lesson  to  some  metallurgists  who 
have  maintained  that  rolls  are  not  adaptable  to  hard  ores.  Some  of  the 
roll  installations  here  are  crushing  extremely  hard,  close-grained  ores  and 
doing  it  efficiently  and  at  low  cost.  The  controversy  as  to  the  supremacy 
of  rolls  or  stamps  might  receive  considerable  light  if  a  thorough  comparison 
of  the  different  practices  in  the  camp  were  made.  The  greatest  difficulty 
is  that  only  one  company,  the  Homestake,  is  using  stamps,  and  this  is 
on  such  an  enormous  scale  that  the  extremely  low  costs  obtained  there 
cannot  be  compared  with  those  obtained  by  rolls  on  a  much  smaller  scale. 
The  Homestake  mills  are  treating  more  than  4000  tons  daily  and  by. 
reason  of  the  extent  of  the  operation  are  able  to  institute  economies 
which  would  be  impossible  in  smaller  plants,  the  largest  of  which  treats 
but  500  tons  daily.  Comparison  of  results  without  a  long  period  of 
study  and  analysis  is  obviously  out  of  the  question. 

High-speed  Chilean  mills  are  operated  in  a  number  of  plants  and  are 
giving  good  results.  I  have  for  some  time  believed  that  the  slow-speed 
mill  is  capable  of  giving  more  economical  results  and  a  comparison  of  the 
cost  of  these  and  the  cost  obtained  by  the  slow-speed  mills  at  the  Minne- 
sota mill  would  be  of  greatest  interest.  In  view  of  the  fact  that  the  latter 
will  probably  begin  operations  soon  I  hope  to  see  this  comparison  made. 

The  use  of  lime  is  accompanied  by  some  losses  which  might  possibly  be 
avoided  by  some  slight  change  of  method.  This  is  typified  in  the  case 
of  the  Wasp  No.  2  which  has  already  been  mentioned  and  a  similar 
procedure  is  followed  at  some  of  the  other  mills.  The  use  of  lime  should  be 
carefully  watched,  as  it  has  been  clearly  proved  at  the  Homestake  that 
excessive  lime  has  a  retarding  effect  on  the  solution  of  gold  in  the  ores  of 
the  Black  Hills. 

On  the  ores  in  general  it  is  said  to  be  true  that  fine  grinding  increases 
extraction,  and  where  the  ores  have  sufficient  value  to  justify  additional 
expense  it  would  seem  to  be  good  business  to  determine  accurately  at  just 
what  point  grinding  can  be  carried  to  return  the  maximum  economical 
extraction. 


CHAPTER  VII 
THE  LIBERTY  BELL  MILL,  TELLURIDE,  COLORADO 

The  mill  of  the  Liberty  Bell  Mining  Co.  is  situated  near  the  town  of 
Telluride  in  San  Miguel  County,  Colo.  It  is  an  old -mill,  operations 
having  commenced  in  1898  and  continued  up  to  the  present  time  with 
only  a  few  interruptions  due  to  reconstruction  and  improvement  of 
the  plant  and  the  unavoidable  loss  of  time  on  account  of  labor  troubles. 
The  mill  began  operations  with  10  stamps  and  the  scale  has  gradually 
been  increased  until  at  present  there  are  80  stamps  in  operation,  crushing 
a  total  of  about  485  tons  of  ore  per  24  hours. 

Type  of  Ore. — The  ore  consists  of  quartz  or  calcite,  banded,  occurring 
either  alone  or  together,  and  often  having  bunches  of  the  feldspathic 
country  rock.  The  vein  material  is  generally  fractured  and  contains  large 
quantities  of  clay  which  produces  a  colloid  slime  in  the  milling  process. 
The  ore  is  not  hard  and  milling  is  comparatively  simple. 

Cyanidation  was  early  installed.  In  1899  an  experimental  plant  was 
built  and  tested  and,  success  having  been  demonstrated,  a  250-ton  leaching 
plant  was  erected  in  1900.  It  was  soon  shown  that  a  substantial  profit 
could  be  obtained  by  this  method,  although  at  that  time  it  was  about  the 
lowest  grade  of  ore  being  profitably  handled  by  such  process.  Conditions 
were  facilitated  by  the  delivery  of  power  from  the  plant  of  the  Telluride 
Power  Co.,  which  had  just  begun  operations.  All  these  conditions  aided 
in  securing  the  satisfactory  result  which  has  been  continuously  improved 
upon  up  to  the  present  time. 

Ore  Breaking. — The  primary  crushing  plant  is  installed  at  the  mine 
and  consists  of  two  HXl8-in.  Blake  sectional  crushers.  The  ore  is 
delivered  above  the  crushers  on  to  a  grizzly  having  3-in.  openings.  The 
pitch  of  the  grizzly  is  steep,  about  52°,  in  order  to  secure  a  satisfactory 
run  of  the  ore,  which  is  wet,  averaging  in  general  about  8%  moisture. 
The  crushers  break  the  ore  to  the  same  approximate  size  as  the  product 
which  has  passed  the  grizzly,  so  that  the  material  delivered  to  the  mill  is 
sufficiently  small  in  one  direction  to  pass  the  3-in.  opening.  It  should 
be  noted,  however,  that  this  is  a  much  different  thing  from  passing  a  3-in. 
ring,  as  material  having  one  3-in.  dimension,  no  matter  what  its  length 
and  breadth  may  be,  will  go  through  the  grizzly  and  large  slabs  of  rock 
are  frequently  run  into  the  mill. 

The  ore  is  carried  to  the  mill  by  means  by  an  aerial  cable  tramway 

67 


68 


DETAILS  OF  CYANIDE  PRACTICE 


THE  LIBERTY  BELL  MILL,  TELLURIDE,  COLORADO  69 

1|  miles  long.  The  cableway  crosses  a  high  divide  between  the  mine 
and  mill,  which  necessitates  a  more  substantial  construction  than  when 
the  whole  line  is  on  one  approximate  level.  The  highest  point  on  the 
tramway  is  1800  ft.  higher  than  the  mill.  At  the  mill  a  steam-heated 
detention  room  is  provided,  which  is  useful  in  winter  for  thawing  the 
buckets  which  have  been  loaded  between  shifts.  Each  bucket  carries 
about  700  to  800  Ib.  of  ore. 

Amalgamation  in  Cyanide  Solution. — There  are  80  stamps  of  850  Ib. 
each  in  the  mill,  having  a  7-in.  drop  and  106  drops  per  minute.  Four 
of  the  five-stamp  batteries  are  equipped  with  12-mesh  screen,  due  to  a 
difference  in  construction,  but  the  rest  carry  14-mesh  wire  screen.  The 
milling  is  done  in  cyanide  solution  carrying  two  pounds  of  KCN  per  ton. 
The  sodium  salt  is  used  but  the  records  are  kept  in  terms  of  KCN  as  is 
customary  practice  at  present. 

In  front  of  each  five-stamp  battery  is  placed  a  copper  amalgamating 
plate,  4  ft.  7  in.  wide  and  8  ft.  long,  with  a  slope  of  2|  in.  to  each  foot.  Over 
this  plate  the  pulp  passes  and  is  amalgamated.  This  detail  is  worthy  of 
particular  attention,  as  some  authorities  have  maintained  that  amalgama- 
tion cannot  be  successfully  carried  out  in  cyanide  solutions.  It  is  done 
successfully  at  the  Liberty  Bell  mill.  Care  and  attention  are  essential 
to  good  results  and  a  knowledge  peculiar  to  this  particular  work  must  be 
obtained  to  operate  it  successfully. 

The  plates  are  maintained  in  a  rather  "  wet"  state,  for  if  they  are  run 
fairly  dry  as  is  usual  when  milling  in  water,  they  soon  become  too  hard 
and  crusted  to  be  of  any  use  in  recovering  the  gold.  Keeping  the  plates 
wet  avoids  the  excessive  hardening  of  the  amalgam  and  allows  the 
retention  of  a  large  percentage  of  the  gold.  In  view  of  the  fact  that  some 
quicksilver  and  amalgam  may  be  scoured  off  the  plates  while  in  this  wet 
condition,  traps  are  used  to  recover  any  particles  which  may  escape. 
It  is  not  claimed  that  the  extraction  under  these  conditions  is  as  high  as 
when  the  operation  is  carried  out  in  water,  but  the  object  of  the  operation, 
which  is  the  recovery  of  any  coarse  gold  which  might  not  readily  dissolve 
in  cyanide,  is  attained  and  the  system  has  obvious  advantages  over  milling 
in  water  where  cyanidation  is  to  follow. 

When  milling  is  done  in  water  there  is  an  appreciable  amount  of  it 
introduced  into  the  cyaniding  system  and  it  follows  that  an  equal  amount 
must  be  discharged  with  the  residues. 

It  is  almost  impossible  to  get  rid  of  the  residual  moisture  without  losing 
some  cyanide  and  dissolved  metal  along  with  it.  To  avoid  the  discharge 
of  excessive  moisture  with  residues  as  far  as  possible  is  an  important  item 
in  cutting  down  losses  and  by  milling  in  cyanide  solution,  wherever  it  is 
possible,  this  discharge  and  its  valuable  content  is  saved  to  a  large  extent. 

One  development  of  milling  in  water,  where  that  measure  seems 


70  DETAILS  OF  CYANIDE  PRACTICE 

unavoidable,  is  the  system  in  use  at  the  mill  of  the  Smuggler-Union 
company,  where  the  pulp  delivered  to  the  cyanide  plant  has  previously 
been  crushed  and  concentrated  in  water  and  contains  an  enormous  pre- 
ponderance of  the  liquid.  This  pulp  is  here  thickened  and  then  filtered, 
from  which  point  it  begins  cyanide  treatment  containing  a  minimum  of 
moisture.  This  is  a  system  which  cannot  be  generally  recommended  and 
its  use  in  special  cases  will  be  considered  later. 

Classification  and  Concentration. — The  pulp  from  the  batteries  is 
carried  to  Richards  hindered-settling  classifiers,  which  make  a  coarse, 
middling  and  fine  product  in  addition  to  the  slime  overflow.  The  latter 
is  taken  to  6X6-ft.  settling  cones,  the  underflow  of  which  is  concentrated 
on  Wilfley  tables,  the  overflow  going  to  Dorr  thickeners. 

The  underflow  product  from  "the  Richards  classifiers  is  concentrated 
separately  on  Wilfley  tables,  the  tailing  going  to  an  Akins  classifier  and  the 
middling  passing  over  a  Bunker  Hill  screen  which  is  fitted  with  16-mesh 
No.  22  wire  screen.  The  oversize  from  this  screen  also  goes  to  the  Akins 
classifier,  the  undersize  being  concentrated  on  a  Wilfley  table  the  tailing 
of  which  goes  to  the  same  classifier.  The  accompanying  flow  sheet  gives 
the  details  of  the  pulp  flow  and  machinery  in  operation. 

The  Akins  classifier  delivers  the  sand  to  a  5X22-ft.  tube  mill  of  the 
Abbe  tire  type,  the  mill  feed  containing  about  40%  moisture.  The  dis- 
charge from  this  mill  is  raised  to  a  diaphragm  cone  where  a  separation  is 
made,  the  overflow  going  to  a  simple  cone  and  the  underflow  from  both 
of  them  going  to  a  second  tube  mill  identical  with  the  first  one  mentioned. 
The  overflow  from  the  simple  cone,  together  with  the  product  of  the 
second  tube  mill,  is  elevated  to  a  second  series  of  amalgamating  plates, 
and  from  the  plates  to  cones,  the  underflow  being  led  to  Deister  slime 
concentrators  and  the  overflow  to  Dorr  thickeners.  Kidney  pulp  dis- 
tributors are  largely  used  in  the  mill.  These  machines  have  already 
been  described.1 

The  tube  mills  are  of  the  tire  type,  which  is  convenient  in  allowing  any 
desired  size  of  inflow  and  outflow  openings.  In  some  cases  mills  of  this 
type  have  proved  more  or  less  unsatisfactory,  due  to  difficulty  in  keeping 
the  tires  running  true  on  the  supporting  rollers,  but  here  this  difficulty 
has  been  overcome  by  careful  installation  on  heavy,  rigid  concrete  blocks 
and  the  use  of  a  deep  flange.  No  difficulty  is  now  experienced  >and  the 
mills  run  true  and  without  vibration. 

The  mills  are  lined  with  silex  blocks  four  inches  thick  set  on  edge. 
The  lining  lasts  from  nine  to  ten  months  without  renewal  and  the  type 
has  been  found  generally  satisfactory.  In  order,  however,  to  take 
advantage  of  any  economy  which  might  be  available,  experiments  are 
now  being  made  with  a  lining  of  the  Komata  type,  but  as  yet  this  work 
1  Eng.  and  Min.  Journ.,  Nov.  26,  1910,  p.  1046. 


THE  LIBERTY  BELL  MILL,  TELLURIDE,  COLORADO  71 

has  not  been  carried  far  enough  to  give  any  conclusive  results.  Danish 
pebbles  are  used  in  the  mills,  about  130  Ib.  per  mill  per  day  being  the 
consumption.  The  mills  require  about  43  hp.  to  keep  them  moving. 
The  feeders  are  of  the  usual  spiral-dip  type. 

There  are  three  of  these  mills  installed,  but  only  two  are  in  regular 
use,  the  third  being  held  in  reserve.  This  reserve  mill  is  so  arranged  that 
it  can  be  used  in  place  of  either  of  the  other  mills  when  one  is  cut  out  for 
relining  or  repair. 

Agitation  of  the  Slime. — The  Dorr  thickeners  deliver  a  pulp  thickened 
to  about  2  :  1  to  the  Hendryx  agitators,  of  which  there  are  six.  The  pulp 
flow  is  continuous  through  the  Dorr  thickeners  and  the  agitators,  de- 
livering continuously  into  the  equalizer  tank  which  feeds  the  filter.  The 
agitators  were  originally  installed  to  take  advantage  of  the  Hendryx 
specialties  in  cyanide  metallurgy,  but  these  having  been  found  of  ques- 
tionable value,  were  abandoned  and  the  tanks  retained  simply  as  agitators. 
They  agitate  by  elevating  the  material  in  a  central  tube  in  the  tank  by 
means  of  a  propeller  screw.  The  system  is  considered  expensive  of  power 
consuming  7  hp.  each  for  the  tanks,  which  hold  about  33  tons  of  dry 
slime. 

Grinding  Requirements. — Due  to  the  soft  and  clayey  character  of  the 
ore,  extremely  fine  grinding  is  not  necessary.  The  statement  would 
probably  be  more  accurate  were  it  said  that  grinding  the  ore  through 
an  80-mesh  screen  is  sufficient  to  reduce  it  to  such  fineness  that  it  may  be 
treated  as  slime.  The  battery  grinding  alone  produces  a  pulp  of  the 
following  analysis:  On  20  mesh,  2.9%;  on  40  mesh,  20%;  on  60  mesh, 
10.6%;  on  80  mesh,  7.3%;  on  100  mesh,  5.6%;  on  200  mesh,  7.6%  and 
through  200  mesh,  46.6%.  After  regrinding,  the  pulp  which  is  subjected 
to  agitation  treatment  has  the  following  analysis:  On  80  mesh  7.5%;  on 
100  mesh,  4.9%;  on  200  mesh,  14.2%  and  through  200  mesh,  73.4%. 

The  pulp  is  easy  to  keep  in  suspension  and  is  sufficiently  fine  to  give 
the  maximum  economical  extraction.  The  pulp  as  treated  contains 
about  30%  of  colloid  slime  and  one  of  the  principal  problems  is  the 
handling  of  the  product  and  inducing  settlement.  In  order  to  promote 
satisfactory  settlement  a  milk  of  lime  is  prepared  and  added  to  the 
inflow  to  the  Dorr  thickeners,  an  average  of  seven  or  eight  pounds  per  ton 
of  ore  being  required.  The  average  specific  gravity  of  the  dry  slime  is 
about  2.68,  a  figure  which  indicates  no  extreme  either  way,  but  about  an 
average  ore  density. 

Filtering  the  Slime  Pulp. — The  Moore  filter  plant  is  one  of  the  earliest 
examples.  The  tanks  are  of  wooden  construction,  but  are  well  put 
together  and  are  still  in  good  condition.  For  filtering  there  are  four  baskets 
of  66  leaves  each,  the  leaves  measuring  6  X  8  ft.  These  baskets  are  operated 
in  two  groups  of  three  tanks  each,  the  middle  tank  being  the  one  in  which 


72 


DETAILS  OF  CYANIDE  PRACTICE 


loading  is  carried  out  and  the  others  used  for  washes  and  discharging. 
The  cycle  is  as  follows:  Loading,  50  min.;  drying  and  transferring, 
10  min.;  washing,  strong  solution,  15  min.;  washing,  weak  solution,  30 
min.;  discharging,  10  min.;  transferring,  five  minutes. 


•4  Richards 
Hindered  Sett  Una 
Cla<;Kifte         a 


QDDDQDDC 


~8x.4-~3  Amalgam  at  I  net 
2-6'Seitnng 

Cones  Overflo 


l~ ' 9- Dorr  Continuous  PulpThichenen 

t-r* — '•$ — -Tp — ^  ~TZ^I££:  — j — ^ -y_ 


I     Stronq  Solution Decanted  Solution 

I     WeakSolution     - / 

Pulp    


\        /\       /  |  i  i  \ /         •»,  t>   nenaryx  ngira 

\/    \/    l{i:r!*~.        =^_         ^eakSolutior,_S-hraae  in  Series 

?^--^:WrFii"X:rri 


9*SXtf 

*!0"<->oula  '-5 


DIAGRAM  OF  TREATMENT  AT  LIBERTY  BELL  MILL. 

The  thickness  of  the  cake  made  varies  from  seven-eighths  to  one  inch, 
approximating  10  tons  per  basket  of  dry  slime.  The  plant  was  con- 
structed to  treat  a  much  smaller  tonnage  than  is  now  being  put  through 


THE  LIBERTY  BELL  MILL,  TELLURIDE,  COLORADO  73 

it,  but  has  been  forced  up  to  its  present  capacity  by  increasing  the  ef- 
ficiency of  each  operation.  One  of  the  refinements  is  the  installation  of  a 
vacuum  pump  of  high  efficiency  to  expedite  the  filtering.  This  machine 
is  of  the  type  usually  used  for  obtaining  a  vacuum  in  the  manufacture  of 
incandescent  electric  lamps.  The  system  is  a  dry-vacuum  operation,  no 
solution  passing  through  the  vacuum  pump.  The  solution  discharge  is 
into  a  specially  constructed  deep  pit  for  securing  a  barometric  discharge. 

The  loss  in  dissolved  metal  amounts  to  about  three  cents  per  ton  of  dry 
ore  and  the  cyanide  mechanically  lost  to  about  0.3  Ib.  per  ton.  The 
filtered  pulp  has  an  average  dilution  of  2  : 1  and  air  lifts  are  used  to  as- 
sist in  maintaining  an  even  consistency  vertically  in  the  tank.  The 
solids  do  not  settle  rapidly,  as  has  already  been  mentioned,  but  the 
air  lifts  are  used  as  an  additional  assurance  of  homogeneity.  Solution 
under  18  Ib.  pressure  is  used  for  discharging  the  cake  and  the  operation 
is  concluded  with  air  under  10  Ib.  pressure. 

The  filter  leaves  require  acid  treatment  about  every  three  months,  and 
to  facilitate  this  operation  without  delay  of  filtration,  a  separate  basket 
is  always  kept  in  readiness  for  instant  use.  The  basket  requiring  acid 
treatment  is  removed  from  service  and  the  extra  one  immediately  put 
into  action.  A  separate  tank  is  provided  for  containing  HC1  for  treating 
the  leaves,  and  the  cost  of  the  acid-treating  operation  mounts  to  about 
0.6  c.  per  ton  of  ore. 

Precipitation  is  accomplished  by  means  of  zinc  shavings,  using  boxes 
of  the  ordinary  type.  It  is  recognized  that  the  use  of  zinc  dust  offers 
conveniences  and  in  many  cases  economies,  but  the  question  has  been 
given  careful  study  here  without  seeming  to  justify  a  change  of  system. 

The  precipitation  at  the  Liberty  Bell  mill  gives  no  trouble  in  any  way, 
the  only  requirement  being  that  the  clarified  solution  be  allowed  to  pass 
over  the  zinc  at  the  stated  rate  of  about  0.7  ton  per  cu.  ft.  of  zinc  shaving 
per  24  hr.  Precipitation  is  good,  head  solutions  carrying  SI  per  ton  give 
tailing  assays  of  one  to  two  cents  per  ton.  The  solution  going  into  the 
pregnant-solution  tanks  is  metered  by  the  use  of  a  device  similar  to  that 
described  in  the  article  on  the  Hollinger  mill  at  Porcupine,  Ontario. 

The  precipitate  is  melted  in  oil-fired  tilting  furnaces,  one  Steele-Harvey 
and  one  Donaldson  furnace  being  installed.  The  operation  is  extremely 
simple  and  presents  no  difficulties.  The  precipitate  is  lightly  acid  treated,, 
fluxed  and  melted,  the  bullion  obtained  averaging  over  900  fine. 

A  small  blast  furnace  is  used  for  cleaning  up  all  waste  product  such  as 
flue  dust,  slag,  sweepings,  etc.  This  material  is  all  briquetted  with 
Portland  cement  in  such  proportion  that  a  satisfactory  slag  will  result 
and  periodical  runs  are  made  which  eliminate  all  waste  products.  The 
results  are  altogether  satisfactory,  there  being  a  substantial  saving  over 
selling  such  material  to  the  smelters. 


74 


DETAILS  OF  CYANIDE  PRACTICE 


Use  of  Heated  Solutions. — The  heating  of  solutions  has  been  found  of 
assistance  in  the  mill.  This  measure  was  adopted  primarily  for  the  pur- 
pose of  bringing  extractions  up  to  the  normal  point  in  cold  weather,  but 
it  was  considered  advisable  to  continue  to  warm  the  solutions  through- 
out the  year  in  order  to  take  advantage  of  the  additional  recovery  of 
the  silver.  The  effect  upon  the  gold  is  not  noticeable. 

The  matter  of  warming  solutions  is  one  that  has  received  some  atten- 
tion from  metallurgists  in  various  countries  and  in  many  cases  has  not 
shown  any  particular  benefits.  At  any  rate  there  has  been  no  con- 
sistency in  the  results  obtained.  The  silver  mills  in  Mexico,  particularly, 
have  experimented  with  solutions  of  various  temperatures  and  generally 
speaking  the  improvement  has  been  so  small  and  variable  that  benefits 
have  been  considered  not  proven.  In  case  of  the  Liberty  Bell,  the  action 


Pipe  Cap 


•Sample 


Disk  free  fo 

moveonshaff 


Tailing .• 

AUTOMATIC  TAILING  SAMPLER  AT  LIBERTY  BELL  MILL. 


of  the  warm  solution  has  not  been  studied  for  sufficient  time  to  warrant 
final  conclusions.  The  benefit,  if  any,  may  not  be  confined  to  the  dissolving 
effect  of  the  solutions  but  may  extend  to  the  amalgamation  and  con- 
centration, as  will  be  mentioned  later. 

The  ore  delivered  to  the  mill  is  not  regularly  sampled  nor  weighed. 
In  order  to  approximate  the  weight,  a  number  of  the  tram-way  buckets 
are  weighed  each  day  and  the  total  weight  of  ore  delivered  is  calculated 
from  this  information,  together  with  the  number  of  buckets  coming  into 
the  mill.  The  latter  are  counted  automatically  by  means  of  an  electrical 
device  which  records  each  bucket  as  it  enters  the  mill,  the  counter  being 
installed  in  the  mill  office.  No  sample  is  taken  of  the  ore,  the  average 
content  being  obtained  by  summing  the  total  mill  production  and  the 


THE  LIBERTY  BELL  MILL,  TELLURIDE,  COLORADO 


75 


content  of  the  tailing  leaving  the  mill.  The  latter  is  sampled  auto- 
matically by  a  device  invented  and  perfected  by  W.  E.  Tracy,  mill  super- 
intendent. The  sampler  has  been  described  before1  but  a  sketch  of  it  is 
herewith  presented  as  it  is  worthy  of  note. 

It  consists  of  a  disk  mounted  on.  a  shaft  and  moved  by  means  of  a 
worm  gear.  A  pipe  is  fixed  to  the  disk,  the  pipe  having  a  slot  through 
which  the  sample  enters.  The  worm  gear  moves  the  disk  slowly  until  a 
point  is  reached  where  the  weight  fixed  to  the  disk  over-balances  the  pipe 
and  the  disk  swings  around  rapidly,  the  slot  in  the  pipe  passing  the 
tailing  stream  and  taking  a  sample  of  it.  The  momentum  is  sufficient 
to  carry  the  disk  far  enough  to  raise  the  pipe  into  a  vertical  position 
again  and  the  sample  runs  out  of  the  curved  end  of  the  sampling  pipe  into 
a  small  launder  which  is  properly  placed  to  receive  it  and  conduct  it  into  the 
sample  receptacle.  The  movement  is  so  rapid  that  the  solids  in  the  sample 
do  not  settle  and  the  pipe  drains  cleanly.  A  pawl  engaging  a  ratchet  cut 
in  the  disk  prevents  a  return  swing  of  the  disk. 

This  system  of  estimation  of  ore  content  cannot  be  recommended  at  all 
times.  It  may  be  fairly  satisfactory  if  every  care  is  taken  to  avoid  mis- 
haps and  losses  within  the  mill,  but  its  great  weakness  is  that  such  losses 
and  shortages  of  production  cannot  be  readily  detected. 

CHEMICAL  CONSUMPTION  AND  COST  AT  LIBERTY  BELL  MILL 


Material 

Consumption  per 
ton  ore  Ib. 

Cost 

Unit 

Zinc 

0  5 

$0  115 

Ib. 

Lime                                         

7-8 

0.005 

Ib. 

Cyanide  (KCN) 

1.3 

0  215 

Ib. 

Litharge  
Pebbles..  

0.3 
0.54 

0.085 
37.75 

Ib: 
ton. 

The  period  of  agitation  which  the  pulp  receives  in  the  mill  is  about  14 
hr.  in  the  mill  circulation  and  about  12  hr.  in  the  Hendryx  tanks.  The 
consumption  of  chemicals  and  the  cost  per  unit  is  about  as  given  in  the 
accompanying  table. 

Metallurgy  of  the  Ores. — The  metallurgy  of  the  Liberty  Bell  ore  is  not 
complicated,  as  there  is  no  rebellious  element  to  be  reckoned  with.  The 
fact  that  silver  exists  in  sufficient  quantity  to  make  is  extraction  an  object 
introduces  an  element  which  is  somewhat  more  complicated  than  when 
only  gold  is  to  be  extracted.  In  this  case,  however,  the  quantity  of  silver 
is  small  and  strong  solutions  do  not  have  to  be  resorted  to.  A  2-lb.  KCN 
solution  is  used  throughout  the  mill.  The  use  of  a  lead  salt,  however,  has 
been  found  advantageous.  At  first  lead  acetate  was  used,  added  to  the 

1  Trans.  A.  I.  M.  E.,  October,  1911. 


76 


DETAILS  OF  CYANIDE  PRACTICE 


agitators,  but  now  litharge  is  in  regular  use,  the  addition  being  made  to 
the  tube  mills  where  it  is  readily  ground  up  and  put  into  solution. 

The  thorough  system  of  concentration  used  is  based  upon  the  belief 
that  it  does  not  pay  to  attempt  to  extract  the  contents  of  the  sulphides  in 
contact  with  the  remainder  of  the  ore  as  the  strong  solution  required  and 
the  long  time  would  entail  a  loss  of  cyanide  and  cost  of  agitation  out  of 
proportion  with  the  recovery.  By  removing  the  sulphides  the  siliceous 
ore  may  be  treated,  as  has  been  shown,  with  weak  solutions  and  short 
time  of  agitation  and  the  sulphides  may  be  handled  in  some  satisfactory 
way. 

Up  to  the  present  the  sulphides  have  been  sold  to  the  smelters,  but 
experiments  are  under  way  looking  toward  a  cyanidation  of  the  product 
on  the  ground.  While  these  experiments  have  not  been  carried  to  con- 
clusion, it  may  be  said  that  the  indications  are  that  the  process  is  feasible, 
and  if  this  is  true  an  additional  saving  will  be  made  possible.  The  idea  is 
to  treat  the  concentrate  in  a  separate  system  using  stronger  solutions  and 
more  time.  The  residues  from  the  concentrate  .treatment  will  be  thrown 
in  with  the  regular  siliceous  ore  and  will  thus  be  given  an  additional  treat- 
ment, with  the  possibility  that  the  total  recovery,  compared  with  its  cost, 
will  show  a  greater  profit  than  the  method  now  followed  will  produce. 

Special  Metallurgical  Features. — The  two  special  metallurgical 
features,  as  have  been  described,  are  first,  amalgamation  on  plates  in 
cyanide  solutions,  and  second,  the  heating  of  mill  solutions  in  order  to  in- 
crease extraction  of  silver. 

PERCENTAGE  EXTRACTION  AT  LIBERTY  BELL  MILL  DURING  FISCAL 

YEAR  1912 


Gold 

First  half 

Second  half 

Amalgamation  

48.98 

50.20 

Cyanidation               

35.71 

34.40 

Concentration 

7.52 

8.40 

T^fal 

09    91 

OQ  on 

Silver 


Amalgamation                                 

7.12 

11.30 

Cyanidation  
Concentration  '  

37.67 
17.24 

39.70 
21.60 

Total  

62.03 

72.60 

The  first  of  these  items  is  one  that  has  been  called  unsuccessful  and 
impossible  in  many  cases,  but  it  has  here  demonstrated  its  value,  and 
there  can  be  no  reasonable  doubt  that  it  is  useful  in  some  cases.  I  should 


THE  LIBERTY  BELL  MILL,  TELLURIDE,  COLORADO 


77 


consider  it  a  valuable  system  to  adopt  in  cases  where  amalgamation  was 
considered  essential  for  the  purpose  of  recovering  coarse  gold  which  would 
not  dissolve  in  cyanide  solutions  in  any  reasonable  length  of  time, 
and  which  existed  in  such  large  quantities  that  the  regrinding  process 
would  not  reduce  it  to  some  form  readily  dissolved.  It  should  be  pref- 
erable to  milling  in  water,  which  has  manifest  disadvantages. 

Extraction  and  Cost. — Due  to  certain  changes  which  took  place  in 
about  the  middle  of  the  present  fiscal  year  and  the  changes  in  mill  results 
which  followed,  the  ore  content  and  recovery  are  given  as  types  for  these 
periods.  The  ore  content  for  the  first  half  of  the  year  was  0 . 27  oz.  of  gold 
and  three  ounces  of  silver;  for  the  second  half,  0.27  oz.  of  gold  and  two 
ounces  of  silver. 

TABLE  I.  LIBERTY  BELL  MILLING  COSTS 


Labor 

Supplies 

Genera]  mill  labor. 

$0  0901 

$0  5811 

Crushing 

0  0531 

0  0194 

Stamping  
Regrmding  
Settling  and  agitating  
Filtering  
Concentrating  
Amalgamati  ng 

0.0935 
0.0104 
0.0150 
0.0372 
0.0476 
0  0426 

0.0973 
0.0665 
0.0390 
0.0509 
0.0199 
0  0224 

Precipitating  

0.0137 

0.0755 

Total  labor  
Total  supplies.    . 

0.4032 
0  9720 

Total      0.9720 

Total  cost  
Depreciation  
Realization  

1.3752 
0.1200 
0.3100 

Total  mill  cost  of  production  

1.8052 

The  increase  in  silver  extraction  in  the  second  half  is  noteworthy  in 
view  of  the  fact  that  the  ore  content  is  one-third  less,  a  situation  which  is 
opposed  to  general  experience.  There  are  two  factors  which  have  changed, 
either  of  which,  or  both,  may  be  in  part  or  wholly  responsible  for  the  dif- 
ference in  results.  The  first  change,  the  mining  of  the  ore  from  a  different 
and  deeper  level,  may  have  its  effect,  and  the  second  change,  the  increase 
in  solution  temperature,  would  naturally  influence  results  to  some  extent. 

Of  a  total  gain  of  10. 57%  only  2. 03%  is  due  to  improved  recovery  in 
the  cyanide  solution.  The  coincident  alteration  of  two  important  factors 
makes  it  impossible  to  say  without  further  study  just  what  part  each  has 
played  in  producing  the  final  result.  The  temperature  of  the  solution  in 
agitators  and  filter  plant  has  been  raised  to  about  80°  F.  and  that  on  plates, 


78 


DETAILS  OF  CYANIDE  PRACTICE 


concentrators  and  settlers  to  about  70°.  The  cost  of  operation,  based  on 
the  milling  of  104,460  tons  in  seven  months,  is  shown  in  the  accompany- 
ing tables. 

TABLE  II.  DISTRIBUTION  OF  GENERAL  EXPENSES  AT  LIBERTY  BELL 

MILL 


Labor 

Per  ton 

Supplies 

Per  ton 

Superintendence  
Heating  

$0.0268 
0.0129 

Pipe  lines  
Bins  .        .... 

S0.00611 
0.00851 

Electric  plant 

0  0106 

Building 

0  06281 

Lubrication  
Pumping  plant  
Watchman 

0.0034 
0.0212 
0  0083 

Electric  plant  
Pumping  plant  
Fuel  and  heating 

0.0111 
0.0198 
0  0471 

Examination  and  tests 

0  0068 

Tools 

0.00431 

0  2fi  141 

0  0901 

Lime  

0.04261 

Lead  salts  

0.0330 

Light  and  power  
Oil  and  waste  

0.03782 
0.0091 

Assaying  and  melting  
Examination  and  tests  
Miscellaneous  

0.03471 
0.0027 
0.0001 

0.5811 

1  Combined  labor  and  supply. 

2  The  power  in  this  item  is  that  used  in  pumping  between  departments. 


CHAPTER  VIII 
PRACTICE  AT  CRIPPLE  CREEK,  COLORADO 

The  Cripple  Creek  district  of  Colorado  is  one  of  the  most  famous  gold 
camps  of  the  world  and  has,  at  times,  been  called  the  richest  gold  district 
in  existence.  Although  this  can  hardly  be  said  to  be  true  at  present, 
still  the  district  is  of  great  importance  and  continues  to  produce  gold  in 
large  quantities.  Its  minerals  are  too  well  known  to  require  any  detailed 
description  at  the  present  time.  The  occurrence  of  tellurides  of  gold  is 
distinguishing  and  is  the  principal  feature  which  opposes  simple  reduction 
methods. 

The  metallurgy  of  the  ores  of  the  district  has  passed  through  all  the 
changes  common  to  most  of  the  gold  camps  of  the  United  States,  plate 
amalgamation,  chlorination,  concentration  and  finally  cyanidation  having 
been  applied  to  the  minerals  with  varying  degrees  of  success,  not  to 
mention  the  procession  of  patented  and  secret  processes  which  have  made 
one  brief  appearance  and  then  vanished  forever.  At  the  present  time 
cyanidation,  occasionally  alone  but  more  generally  in  combination  with 
concentration  or  some  other  metallurgical  means,  notably  roasting,  is 
applied  to  all  the  ores  which  are  too  low  grade  to  smelt,  and  the  process 
has  become  essential  to  the  life  of  the  district. 

Many  Mills  in  Operation. — There  are  a  number  of  mills  at  work  in  the 
district  and  in  addition  to  these  a  quantity  of  ore  is  shipped  to  the  two 
large  custom  mills  operating  at  Colorado  Springs.  The  richer  ores  are 
smelted  but  the  quantity  of  this  product  is  not  so  important  as  it  was  in 
earlier  days.  By  far  the  greater  quantity  of  ore  is  milled  and  the  cyanide 
process  is  responsible  for  the  recovery  of  a  large  part  of  the  gold. 

Most  of  the  mills  in  the  district,  or  at  any  rate  the  most  important 
ones,  use  a  combination  of  concentration  and  cyanidation,  the  idea  being 
to  remove,  as  far  as  possible,  the  tellurides,  sulphides  and  other  refractory 
compounds  which  do  not  yield  readily  to  cyanide  treatment,  leaving  a 
tailing  which  is  satisfactorily  treated  with  cyanide  solutions  of  low 
strength  and  at  reasonable  cost.  "  Some  of  the  plants  are  simple  leaching 
installations  treating  ore  or  waste  of  low  grade  and  in  different  degrees  of 
fineness,  but  these  present  no  metallurgical  novelties  and  are  simply 
efforts  to  extract  a  profit  from  material  available  without  reference  to 
technical  niceties  or  conservation  of  resources. 

Of  the  more  important  mills  which  are  following  modern  practice  and 
obtaining  results  which  are  satisfactory  in  various  degrees,  may  be  meh- 

79 


80 


DETAILS  OF  CYANIDE  PRACTICE 


PRACTICE  AT  CRIPPLE  CREEK,  COLORADO  81 

tioned  those  of  Stratton's  Independence,  Ltd.,  the  Portland  Gold  Mining 
Co.,  the  Ajax-Colburn  and  the  Blue  Flag  Mining  Co.  Of  these  the 
Ajax  is  the  principal  one  which  has  made  serious  attempts  to  obviate  the 
use  of  both  concentration  and  roasting  and  use  the  cyanide  process 
direct,  making  use  of  chemical  means  in  combination  with  the  cyanide 
solutions  to  dissolve  the  refractory  gold  compounds.  This  mill  has 
installed  the  Clancy  electrochemical  method  of  procedure  and  is  now  in 
process  of  ascertaining  what  benefits  may  be  expected  from  it.  At  pres- 
ent there  is  not  sufficient  information  available  to  enable  a  statement 
which  will  definitely  prove  either  success  or  failure  for  the  system,  but 
it  is  hoped  that  within  a  few  months  data  for  the  settlement  of  the  question 
may  be  forthcoming.  The  serious  attempt  of  the  operators  of  this  plant 
to  solve  a  difficult  problem  along  scientific  lines  is  worthy  of  the  complete 
success  that  they  hope  to  obtain. 

Separate  Dry  Crushing  at  Ajax. — The  Ajax  has  installed  a  plant 
entirely  separate  from  its  cyanide  mill  in  which  the  ore  is  crushed  dry  and 
mechanically  sampled.  The  ore  is  brought  into  this  plant  in  electrically 
moved  cars  and  is  passed  through  a  series  of  crushers,  rolls,  screens,  etc., 
being  sampled  during  the  process,  and  is  finally  delivered  in  a  condition 
sufficiently  fine  to  be  mixed  with  the  cyanide  solutions  and  classified,  any 
oversize  being  led  directly  into  the  tube  mills  for  regrinding.  This,  I 
believe,  is  the  first  attempt  in  the  Cripple  Creek  region  to  slime  all  the 
milling  ore  and .  treat  it  directly  by  cyanide.  The  statement  of  proven 
results  will  be  awaited  with  interest. 

The  Blue  Flag  mill  is  another  which  is  making  a  trial  of  a  compara- 
tively new  development  of  cyanidation.  It  is  equipped  with  machinery 
for  the  continuous-decantation  process  using  Dorr  thickeners  in  con- 
junction with  agitation  tanks  of  a  well-known  design.  This  process 
presents  some  innovations  which  are  of  great  interest  at  this  time  and 
will  be  discussed  in  a  separate  article. 

Portland  and  Independence  Mills. — The  mills  of  the  Portland  Gold 
Mining  Co.  and  Stratton's  Independence,  Ltd.,  are  similar  in  many  ways, 
the  principal  difference  being  that  the  Portland  makes  a  total  slime  prod- 
uct, while  at  the  Independence  the  sand  and  slime  are  treated  separately. 
Due  to  the  fact  that  the  Portland  company  does  not  desire  to  make  public 
any  details  of  its  practice  at  the  present  time,  no  information  can  be  given, 
but  it  will  be  safe  to  accept  the  practice  followed  at  the  Independence  as 
typical  of  successful  work  in  the  Cripple  Creek  district  and  its  system, 
which  is  made  public  without  reserve,  will  form  the  bulk  of  this  article. 

At  this  mill  a  long  series  of  experiments  was  conducted  by  Philip 
Argall  and  it  was  decided  that  the  most  feasible  method  was  careful  con- 
centration of  the  ores  followed  by  cyanidation  of  the  tailing.  The  adapta- 
bility of  this  system  was  due  to  the  fact  that  the  ore  contained  a  quan- 


82 


DETAILS  OF  CYANIDE  PRACTICE 


PRACTICE  AT  CRIPPLE  CREEK,  COLORADO  83 

tity  of  gold  in  the  form  of  telluride  which  is  not  easily  cyanied  by  direct 
methods  and  it  was  considered  advisable  to  remove  this  constituent  and 
extract  its  gold  content  by  some  means  more  satisfactory.  A  mill  to 
treat  5000  tons  per  month  was  erected  to  make  use  of  this  method.  The 
mill  was  afterward  increased  to  handle  10,000  tons  per  month,  at  which 
capacity  it  is  now  operating. 

The  ore  supply  comes  mainly  from  the  dumps,  a  large  accumulation 
of  this  material  having  resulted  during  the  long  life  of  the  mine.  The 
average  content  has  a  value  of  about  $3 . 50  per  ton  as  milled.  An  elec- 
tric shovel  is  used  for  moving  the  material,  which  is  loaded  into  4-ton  cars, 
hoisted  up  an  inclined  plane  and  delivered  into  the  crusher  house. 

COSTS  AT  STRATTON'S  INDEPENDENCE  MILL 

Per  ton 
treated 
Dump  breaker: 

Power $0.0436 

Operation 0.0516 

Repairs. 0.0681 

Total $0. 1633 

Crushing  and  concentrating: 

Power 0. 1946 

Operation 0.1192 

Repairs 0. 1982 

Loading  concentrate 0 . 0097 


Total •      0.5217 

Cyaniding: 

Power 0.0484 

Operation 0.3420 

Repairs 0.0549 


Total ' 0.4453 

Miscellaneous: 

Heating 0.0050 

Water  service 0.0048 

Liability  insurance 0 . 0057 

'Fire  insurance 0.0298 

Taxes..  0.0465 


Total 0.0918 

Mine  breaker1 0.0169 

Total  cost $1 .2390 

1  The  mill  receives  one-tenth  of  its  supply  from  a  small  breaker  plant  where  mine 
ore  is  crushed  to  \  in.  and  taken  directly  to  the  Chilean  mills. 


84  DETAILS  OF  CYANIDE  PRACTICE 

From  the  head  of  the  crusher  house  the  ore  is  delivered  to  a  No.  7  J 
Gates  gyratory  crusher,  which  delivers  its  crushed  product  to  a  picking 
belt  three  feet  wide.  Here  the  larger  boulders  of  barren  rock  are  thrown 
out  as  much  as  possible  and  the  belt  carries  the  remaining  material  to  a 
No.  5  Gates  crusher,  in  which  the  ore  is  reduced  to  about  1  f-in.  cubes 
and  conveyed,  by  means  of  an  18-in.  conveying  belt,  to  the  steel  storage 
bin.  The  sorting  does  not  materially  increase  the  value  of  the  ore,  less 
than  3%  being  removed  in  the  process,  which  is  of  more  importance  on 
account  of  the  quantity  of  pieces  of  wood,  steel  and  much  other  foreign 
matter  which  comes  from  the  dump. 

The  steel  mill  bin  is  of  special  design  and  was  installed  with  the  idea 
of  obviating  blocking  or  sticking  of  the  ore,  which  object  has  been  attained. 
It  consists  of  a  steel  cylinder,  the  bin  proper,  terminating  in  a  cone-shaped 
bottom.  The  apex  of  the  cone  is  cut  off  leaving  a  circular  opening  four 
feet  in  diameter.  This  opening  is  set  down  into  another  smaller  cone,  the 
feed  cone,  which  has  a  12-in.  opening  delivering  the  ore  to  a  revolving 
disk  feeder.  The  opening  in  the  bin  bottom  is  large  enough  to  prevent 
blocking  and  is  small  enough  to  relieve  the  feed  cone  of  excessive  pressure. 
The  device  works  successfully  and  the  bin  can  be  entirely  emptied  of  its 
contents  without  shoveling.  The  ore  stored  in  the  bin  is  all  reduced  to 
smaller  than  2-in.  cubes. 

From  the  storage  bin  the  ore  is  fed  through  two  sets  of  16X36-in. 
rolls  which  reduce  it  to  about  J-in.  size.  Due,  however,  to  the  flaky 
character  of  the  ore,  largely  phonolite,  the  size  is  more  nearly  f  in., 
although  its  thickness  is  generally  less  than  J  in. 

Addition  of  Lime. — The  ore  coming  from  the  storage  bin  is  slightly 
moistened  with  cyanide  solution  which  tends  to  settle  the  dust  and  also 
to  slake  the  lime  which  is  added  at  this  point.  By  means  of  this  procedure 
it  is  calculated  that  the  lime  shall  be  in  good  condition  to  render  effective 
service,  and  by  the  passage  through  the  rolls  normal  character  of  ore  feed, 
they  crush  almost  exactly  five  tons  per  hour  with  an  expenditure  of  55  hp. 
With  the  mill  in  average  condition  the  pulp  delivered  from  it  has  the  fol- 
lowing approximate  composition:  On  50  mesh,  21%;  on  100  mesh,  12%; 
on  150  mesh,  5%;  through  150  mesh,  62  per  cent. 

The  total  steel  consumed  per  ton  of  ore  milled,  using  Midvale  or 
Latrobe  brands,  amounts  to  0.62  Ib.  The  crushing  pressure  of  the' mill 
amounts  to  about  900  Ib.  per  sq.  in.,  which  is  probably  somewhat  increased 
by  centrifugal  force. 

The  Chilean  mill  has  been  generally  considered  a  sliming  machine, 
but  the  operators  at  the  Independence  mill  have  found  that  within  certain 
limits  it  is  able  to  give  a  fine,  yet  granular  product.  The  adjustment, 
however,  must  be  made  with  a  view  to  the  result  desired.  In  some  cases, 
of  which  the  Independence  is  typical,  a  fine,  granular  product  is  desired 


PRACTICE  AT  CRIPPLE  CREEK,  COLORADO  85 

for  concentration  efficiency,  while  in  other  instances,  typified  by  the  total- 
sliming  mills,  the  highest  economy  is  secured  by  producing  an  extremely 
fine  product  as  quickly  and  simply  as  possible.  In  the  latter  case  the 
Chilean  mill  has  been  able  to  prove  its  adaptability,  and  when  construc- 
tion includes  the  Mantey  offset  I  should  be  inclined  to  consider  the  large 
diameter,  slow-speed  mill  both  cheaper  and  more  efficient  than  the  higher- 
speed  mills  such  as  are  used  at  the  Independence.  In  connection  with 
this  subject  it  will  be  interesting  to  refer  to  my  article  on  Chilean  mills1 
and  also  the  paper  by  Walter  H.  Urbiter2  on  the  same  subject. 

The  ore  coming  from  the  storage  bin  is  slightly  moistened  with 
cyanide  solution  which  tends  to  settle  the  dust  and  also  to  slake 
the  lime  which  is  added  at  this  point.  By  means  of  this  procedure 
it  is  calculated  that  the  lime  shall  be  in  good  condition  to  render 
effective  service,  and  by  the  passage  through  the  rolls  becomes 
thoroughly  mixed  in  with  the  ore  and  probably  reaches  every  part 
of  it.  Being  in  a  damp  condition,  the  time  of  passage  through  the  rolls 
to  the  mill  bins  and  the  time  it  remains  in  the  latter  should  be  sufficient 
for  a  thorough  neutralization  of  any  latent  or  developed  acidity  in  the  ore. 
The  system  of  adding  lime  to  ores  in  cyanide  milling  has  been  the  subject 
of  much  discussion  and  the  methods  used  are  extremely  varied.  It  is 
of  considerable  interest  to  note  the  methods  and  reasons  adopted  at  the 
different  plants,  and  a  final  comparison  of  them  will  be  valuable  and  of 
technical  interest. 

Chilean  Mills  for  Fine  Crushing. — From  the  rolls  the  ore  is  taken  to 
the  storage  bins  above  the  Chilean  mills,  which  are  used  for  fine  crushing. 
The  fine-crushing  plant  consists  of  four  6-ft.  Akron  Chilean  mills.  Three 
of  these  are  kept  in  operation  all  the  time,  easily  crushing  the  required 
amount  of  10,000  tons  per  month,  the  third  being  held  in  reserve  and 
ready  for  use  at  any  time.  The  mills  can  each  crush  from  100  to  130  tons 
per  day,  according  to  the  feed  they  receive.  The  ore  is  fed  into  them  by 
means  of  revolving-disk  feeders  which  may  be  adjusted  to  any  required 
capacity. 

The  mills  are  fitted  with  square  wire  screen  having  0.046-in.  aperture 
and  0.054-in.  wire.  The  speed  is  about  33  r.p.m.,  and  with  this  adjust- 
ment, using  the  normal  character  of  ore  feed,  they  crush  almost  exactly 
five  tons  per  hour  with  an  expenditure  of  55  hp.  With  the  mill  in  aver- 
age condition  the  pulp  delivered  from  it  has  the  following  approximate 
composition:  On  50  mesh,  21%;  on  100  mesh,  12%;  on  150  mesh,  5%; 
through  150  mesh,  62  per  cent. 

The  total  steel  consumed  per  ton  of  ore  milled,  using  Midvale  or  Lat- 
robe  brands,  amounts  to  0.62  Ib.  The  crushing  pressure  of  the  mill 
amounts  to  about  900  Ib.  per  sq.  in.,  which  is  probably  somewhat  increased 
by  centrifugal  force. 

1  "Eng.  and  Min.  Journ.,"  Nov.  12,  1910. 

2  "Eng.  and  Min.  Journ.,"  Aug.  5,  1911. 


86 


DETAILS  OF  CYANIDE  PRACTICE 


Concentration  a  Feature. — From  the  Chilean  mill  the  pulp  is  led  to  the 
Ovoca  classifiers,  which  are  double  spiral-screw  machines  designed  by 
Philip  Argall.  The  milling  is  done  in  cyanide  solution  containing  \  Ib. 


KCN  per  ton,  and  from  that  point  the  ore  is  constantly  in  solution.  The 
Ovoca  classifiers  give  a  sand  practically  free  from  slime  with  from  15  to 
25%  moisture,  thus' performing  the  operations  of  separation  and  sand 


PRACTICE  AT  CRIPPLE  CREEK,  COLORADO  87 

de watering.  The  sand  is  sent  to  20  Card  concentrators.  There  are  22 
installed  and  the  pulp  is  distributed  to  the  tables  by  means  of  an  auto- 
matic distributor  which  has  been  perfected  in  use  at  this  mill. 

The  slime  from  the  classifier  is  thickened  in  cones  and  sent  to  the  slime 
concentration  department,  which  is  equipped  with  13  Deister  tables  and 
four  vanners.  The  Deister  slime  concentrator  here  performs  equally  as 
good  work  as  the  vanners  and  the  maintenance  expense  is  less  than  one- 
tenth  of  that  of  the  vanners. 

From  the  sand  concentrators  two  products  are  obtained,  a  high-grade 
concentrate,  which  is  shipped  direct,  and  a  middling,  which  is  reground 
and  reconcentrated.  Originally  it  was  designed  to  treat  the  concentrate 
on  the  ground  and  a  roasting  plant  for  that  purpose  was  erected,  but  it 
was  finally  decided,  on  account  of  the  character  of  the  concentrate,  which 
is  desired  by  the  smelters,  to  ship  it.  The  concentrate  can  now  be  con- 
sidered as  having  its  gold  content  definitely  recovered  as  the  other  ele- 
ments contained  pay  all  its  expenses,  leaving  the  gold  net.  Under  such 
circumstances  it  would  not  be  wise  to  attempt  its  local  treatment. 

High  Value  of  Finer  Material. — In  the  case  of  the  Independence  ores 
it  is  invariably  true  that  the  finest  material  carries  the  most  gold.  The 
gold  occurs  mainly  in  films  along  the  fracture  planes  or  in  small  cavities, 
the  sulphide  in  the  body  of  the  rock  being  of  low  grade  and  often  worth- 
less. It  is  due  to  these  basic  facts  that  the  crushing  system  in  use  was 
devised  and  followed,  it  being  briefly  an  attempt  to  liberate  the  sulpho- 
tellurides  to  sufficient  extent  that  they  might  be  removed  by  concentra- 
tion, leaving  a  sand  tailing  of  low  grade  that  might  be  lightly  cyanided 
and  discarded. 

The  slime,  containing  the  higher  value,  is  also  concentrated  as  closely 
as  possible  and  cyanided  carefully  by  more  efficient  methods.  .The  slime 
tailing  has  never  been  reduced  as  low  as  the  sand.  The  guide  now  chosen 
is  to  keep  the  sand  tailing  below  $1  per  ton  in  value,  making  a  slime  that 
runs  in  the  neighborhood  of  $2  per  ton  after  concentration.  The  coarse 
sand  produces  a  high-grade  concentrate,  running  from  five  to  seyen  ounces 
per  ton,  a  middling  which  is  reground,  in  turn  producing  rich  concentrate 
and  a  proportion  of  slime,  and  a  tailing  of  extremely  low  gold  content,  as 
has  been  mentioned.  This  procedure  enables  the  comparatively  coarse 
crushing  to  be  practised  in  the  Chilean  mills  and  renders  obtainable  a 
crushing  cost  which  would  be  out  of  the  question  were  total  sliming  re- 
sorted to. 

Sand  Leaching. — The  sand  and  slime  tailings  are  both  pumped  to  the 
cyanide  plant  where  a  pair  of  Ovoca  classifiers  separate  them  finally,  the 
sand  being  moved  by  a  special  conveyor  into  the  sand-leaching  tanks. 
This  conveyor  is  the  usual  "grasshopper"  type,  a  long  bar,  supported  on 
wheeled  trucks,  having  a  number  of  hinged  pusher  blades  working  in  a 


88  DETAILS  OF  CYANIDE  PRACTICE 

trough  or  launder.  The  launder  is  arranged  so  that  openings  can  be 
made  into  each  tank  at  several  different  places.  The  sand  is  distributed 
practically  automatically,  the  final  leveling  of  the  top  of  the  tank  being 
about  the  only  hand  labor  required.  A  stream  of  solution  is  led  into  the 
tank  along  with  the  sand,  which  is  so  free  from  slime  that  no  trouble  with 
channeling  or  segregation  of  slime  is  experienced. 

The  sand  thus  laid  into  the  tanks  is  treated  with  i-lb.  cyanide  solu- 
tion for  four  days  and  is  then  sluiced  out  and  run  to  waste.  The  sand 
treatment  is  extremely  simple;  but  since  the  precious-metal  content  is 
small,  the  economical  maximum  of  extraction  is  reached  in  this  way. 

Bromocyanidation  of  Slime. — The  separated  slime  from  all  depart- 
ments is  led  into  four  continuous  thickening  tanks,  the  products  being  a 
clear  overflow  and  a  thick  slime  which  is  pumped  into  a  treatment  tank 
where  it  receives  a  six-hour  treatment  with  f-lb.  cyanide  solution,  and 
afterward  a  four-hour  treatment  with  bromocyanide  in  a  separate  tank. 
The  bromocyanide  treatment  is  in  accordance  with  the  general  practice, 
the  solvent  being  made  from  the  usual  "miners  salt"  in  the  ordinary  way. 
The  only  departure  from  generally  accepted  practice  is  that  it  is  found 
necessary  to  maintain  a  comparatively  high  alkalinity  during  the  treat- 
ment, general  practice  insisting  on  a  neutral  solution  in  most  cases. 

The  tanks  used  for  agitation  are  a  special  design,  the  features  of  which 
are  a  pointed  cone  bottom  and  a  cylindrical  section  of  little  height,  a  cen- 
tral agitation  pipe,  into  which  air  is  admitted  at  the  bottom,  and  having  a 
cone  over  its  outlet,  which  is  in  about  the  middle  of  the  vertical  height  of 
the  tank.  The  cone  is  open,  with  its  vertex  pointed  down  and  is  designed 
to  spread  the  outcoming  pulp  over  the  area  of  the  tank.  It  is  simply 
another  method  of  taking  advantage  of  pneumatic  agitation,  the  varia- 
tions of  which  are  without  number.  The  accompanying  flow  sheet  shows 
the  movement  of  pulp  and  solution  and  the  machinery  installed.  There 
is  little  novelty  in  the  methods  used  in  cyaniding  the  concentrate  tailing. 

The  pulp  is  fed  from  a  storage  tank  into  a  vacuum  filter  of  the  station- 
ary or  Butters  type,  in  which  the  leaves  are  fixed  and  the  movement  is  of 
pulp  and  solutions.  This  filter  is  built  after  the  Cassell  design  and  was 
one  of  the  first  of  the  kind  to  be  used.  It  shows  its  early  construction, 
but  is  ^still  doing  efficient  work.  The  leaves  are  made  of  canvas  with  a 
filling  of  cocoa  matting,  and  are  stitched  with  wire  instead  of  the  usual 
thread.  This  is  said  to  prevent'ripping  and'obviate  a  large  portion  of  the 
usual  repair  bills.  Solutions  for  precipitation  are  clarified  by  passing 
them  through  a  filter  press  of  the  plate-and-frame  type. 

Precipitation  on  Zinc  Shavings. — Precipitation  is  in  the  ordinary  man- 
ner by  the  use  of  zinc  shavings.  There  is  little  difficulty  with  the  opera- 
tion. The  barren  solution  produced  is  used  first  on  the  concentration 
tables  as  wash.  The  precipitate  is  not  melted  or  refined  at  the  plant,  it 


PRACTICE  AT  CRIPPLE  CREEK,  COLORADO  89 

having  been  proved  more  economical  to  ship  it  to  smelters  for  final  treat- 
ment. The  precipitate  is  thoroughly  dried,  sampled  and  sealed  up  in  cans 
for  shipment.  The  fact  that  precipitate  of  this  class  can  be  shipped  at  a 
less  cost  than  treating  it  on  the  ground  is  worthy  of  comment,  for  it  is 
generally  considered  better  practice  to  handle  it  at  the  plant,  where  there 
is  less  chance  for  loss  due  to  differences  of  sampling,  to  say  nothing  of 
transportation  charges.  This  matter  ought  to  be  investigated,  particu- 
larly by  plants  situated  conveniently  to  smelting  installations,  as  I  be- 
lieve that  generally  no  investigations  are  seriously  made,  it  being  taken 
for  granted  that  treatment  on  the  ground  is  cheaper. 

Extraction  and  Consumption  of  Material. — The  extraction  totals 
71.5%  of  the  content,  of  which  43.65%  is  obtained  by  concentration  and 
27.85%  by  cyanidation.  The  consumption  of  chemicals  is  about  0.45  Ib. 
of  NaCN  per  ton,  0.3  Ib.  of  zinc,  2  2  Ib.  lime  and  0.4  Ib.  of  the  bromine 
salt.  The  mill  consumes  about  320  hp.  and  employs  35  men  regularly  in 
the  operations.  The  costs  are  moderate  for  the  method  followed  and  are 
shown  in  the  accompanying  table. 

[DISCUSSION 

The  Shipment  of  Cyanide  Precipitate. — In  the  JOURNAL  of  Feb.  8, 
1913,  Mr.  Megraw  comments  on  the  shipment  of  cyanide  precipitate -to 
smelters  as  being  cheaper  than  home  treatment,  and  advises  other  plants 
to  investigate  the  matter.  However,  he  seems  to  state  the  vital  objec- 
tion to  it  in  the  same  paragraph,  "the  chance  of  loss  due  to  difference  in 
sampling." 

I  have  seen  this  thing  tried  out  where  I  knew  that  every  effort  was 
being  made  to  give  the  shipper  a  square  deal,  and  as  a  check,  the  shipper 's 
material  was  treated  by  itself,  as  far  as  producing  bullion  and  byproducts 
went.  The  bullion  alone  carried  more  metal  than  the  assays  showed  in  the 
original  precipitate,  while  there  was  a  whole  string  of  metalliferous  by- 
products still  to  be  heard  from. 

Of  course  the  balance  might  be  in  favor  of  the  shipper,  I  remember 
hearing  that  one  of  the  difficulties  of  the  old  Seattle  Smelting  &  Refining 
Co.  was  due  to  erroneous  sampling  on  exceedingly  high-grade  material, 
giving  the  shippers  more  than  was  coming  to  them,  but  usually  the 
sampler  will  err,  although  perhaps  unconsciously,  on  his  own  side. 

Considering  the  expense  of  tight  coopering  or  bagging,  freight,  the 
danger  of  theft  in  transit,  and  errors  in  sampling  and  assaying,  it  seems 
that  a  plant  shipping  cyanide  precipitate,  silver  sulphides,  or  like  material, 
is  taking  a  long  chance. 

K.  T.  WHEELER. 

Pittsburgh,  Penn.,  Feb.  10,  1913. 


90  DETAILS  OF  CYANIDE  PRACTICE 

In  the  JOURNAL  of  Feb.  8,  1913,  Mr.  Megraw,  in  his  excellent  article 
upon  "Cyanidation  at  Cripple  Creek,"  refers  to  the  practice  pursued  at 
the  Independence  mill  of  shipping  the  precipitate  to  smelters,  saying: 
"The  fact  that  precipitate  of  this  class  can  be  shipped  at  a  cost  less  than 
that  for  treating  it  upon  the  ground  is  worthy  of  comment,  for  it  is  gen- 
erally considered  better  practice  to  handle  it  at  the  plant  where  the  chance 
of  loss,  due  to  differences  in  sampling,  is  less,  to  say  nothing  of  transporta- 
tion charges.  This  matter  ought  to  be  investigated,  particularly  by 
plants  situated  conveniently  by  smelting  installations,  as  I  believe  that 
generally  no  investigations  are  seriously  made,  it  being  taken  for  granted 
that  treatment  on  the  ground  is  cheaper." 

In  commenting  upon  this  feature  of  Mr.  Megraw's  article,  Mr. 
Wheeler,  in  the  JOURNAL  of  Feb.  15,  1913,  doubts  the  wisdom  of  this 
course  for  various  reasons,  although  he  gives  no  definite  figures  to  support 
his  opinion.  Surely  so  experienced  an  operator  as  Mr.  Argall  is  not 
shipping  his  precipitate,  in  preference  to  local  reduction,  without  good 
reason.  The  fact  that  this  plant  is  situated  convenient  to  smelteries,  and 
that  gold  precipitate  is  produced,  may  have  its  influence.  In  general, 
smelters  do  not  buy  this  material  upon  the  result  of  corrected  assays,  and, 
as  it  is  a  well  known  fact  that  the  percentage  of  correction  upon  the  assay 
of  a  product  carrying  largely  gold  is  not  so  great  as  upon  one  in  which  sil- 
ver predominates,  it  will  be  readily  seen  that  the  difference  in  favor  of  the 
smelter,  arising  from  this  cause,  would  not  be  so  great  as  in  the  latter  case. 

The  transportation  problem  is  a  serious  one  and,  perhaps,  is  at  its 
worst  when  long  distances  have  to  be  covered  by  mule  back  or  ox  cart  and 
steamer.  I  have  seen  various  methods  of  packing  tried,  such  as  sacking 
in  canvas  and  burlap,  paper  sacks  inside  of  canvas  protected  by  burlap, 
boxing,  etc.,  but  the  only  satisfactory  method  is  to  pack  it  in  hermetically 
sealed  tin  cans  protected  by  wooden  cases.  A  satisfactory  container  for 
this  purpose  can  be  readily  obtained  at  any  cyanide  plant  by  carefully 
opening  and  preserving  the  tin-lined  wood  cases  in  which  the  cyanide  is 
received.  These  should  be  washed  and  dried.  The  precipitate  is  then 
placed  in  them,  the  tin  cover  soldered  on,  and  the  wood  top  securely 
fastened  by  nails  or  screws.  A  binding  of  light  strap  iron  greatly 
strengthens  the  package  and  is  desirable  if  it  is  to  go  any  distance. 

Cyanide  operators  would  welcome  comparative  figures  from  Mr. 
Argall,  showing  the  advantage  of  shipping  precipitate  instead  of  bullion. 
Various  plants,  with  which  I  have  been  connected  in  the  past,  have 
shipped  precipitate  and  then  have  later  installed  melting  furnaces  and 
shipped  bullion.  In  every  case  it  was  felt  that  there  was  a  distinct  advan- 
tage, although  no  accurate  comparisons  were  made,  in  favor  of  producing 
bullion,  and  in  no  case  did  any  of  these  plants  return  to  shipping  precipi- 
tate after  having  once  produced  bullion. 


PRACTICE  AT  CRIPPLE  CREEK,  COLORADO  91 

A  number  of  years  ago  I  had  an  opportunity  of  making  a  thorough  and 
fair  comparison  between  the  relative  merits  of  shipping  precipitate  and 
the  production  and  shipping  of  bullion,  when  Charles  Butters  requested 
me  to  go  to  Virginia  City,  Nev.,  and  thoroughly  investigate  this  feature 
of  their  practice.  At  this  time  the  Virginia  City  plant  was  treating  a 
considerable  tonnage  of  Tonopah  ore  and  old  Comstock  tailing.  Both 
electrical  and  zinc  precipitation  were  in  use  and  the  precipitate  resulting 
from  the  two  methods  was  quite  different  in  character.  The  electrically 
precipitated  material  was  high  in  lime  and  copper  and  also  contained 
some  lead.  The  zinc-box  precipitate  was  similar  to  that  produced  in 
plants  employing  that  method  of  precipitation  when  treating  a  silver-gold 
ore,  except  that  it  was  not  quite  so  high  grade  as  that  produced  by  present- 
day  practice.  The  precipitate  from  the  electric  boxes  formed  the  bulk  of 
the  product.  Both  classes  of  precipitate  contained  mercury  and  it 
was  the  practice  to  retort  for  its  recovery  until  the  Comstock  tailing, 
which  was  its  source,  finally  became  exhausted,  when  retorting  was 
discontinued. 

From  the  beginning  of  operations  at  this  plant  it  had  been  the  practice 
to  ship  the  precipitate,  and  this  was  continued  long  after  the  other  Butters 
plants  had  found  it  to  their  advantage  to  ship  bullion.  It  was  generally 
supposed,  owing  to  the  refractory  nature  of  the  precipitate,  and  its  low 
gold  and  silver  content,  that  there  was  every  advantage  in  favor  of  ship- 
ping precipitate  rather  than  bullion. 

I  have  no  reason  to  doubt  the  accuracy  of  the  sampling  and  assaying 
of  the  smelters  to  whom  the  precipitate  was  usually  shipped,  as  their 
results  generally  agreed  with  the  independent  sampling  and  assaying  at 
the  mill;  but,  of  course,  it  is  to  be  understood  that  neither  party  made 
corrected  assays.  It  will,  therefore,  be  seen  that  the  factor  of  dealing 
with  an  unscrupulous  smelter  does  not  necessarily  enter  into  the  case  in 
hand. 

For  the  purpose  of  experimentation  there  was  erected,  under  my  direc- 
tion, a  single  stationary  pot  furnace  and  a  small  cupeling  furnace.  It  was 
planned  to  carry  on  experiments  to  determine  the  relative  advantages  of 
fluxing  and  melting  directly  in  graphite  crucibles,  producing  a  bullion 
containing  most  of  the  copper;  and  melting  in  the  cupel  furnace  with 
litharge  and  other  fluxes,  running  off  the  slag  and  cupeling  the  lead,  thus 
producing  a  high-grade  bullion.  A  number  of  experiments  were  carried 
out  along  both  lines  and  it  was  finally  decided  that  it  would  be  more 
advantageous  to  adopt  the  method  of  melting  in  pots. 

The  results  obtained  with  the  cupel  furnace  all  showed  a  substantial 
gain  over  shipping,  but  as  the  pot  method  was  the  one  finally  adopted  it 
will  suffice  to  give  comparative  figures  for  that  method  and  shipping. 
For  that  purpose  I  will  give  the  results  of  two  experiments  made  upon  two 


92  DETAILS  OF  CYANIDE  PRACTICE 

samples  taken  from  the  precipitate  comprising  two  separate  and  distinct 
shipments  to  the  smelter. 

These  large  samples  were  taken  at  the  time  that  the  precipitate  was 
being  sampled  for  assay  prior  to  shipment.  Every  precaution  was  taken 
to  have  them  truly  representative  of  the  mass  of  the  material  shipped,  and 
in  my  opinion  they  were  even  more  representative  of  the  whole  mass  than 
the  small  samples  taken  independently  at  the  mill  and  smeltery  for  assay. 
Assays  upon  these  independent  samples  generally  checked  closely. 

In  the  first  shipment  the  fine  ounces  of  silver  and  gold  realized  by 
smelting  460  Ib.  of  precipitate  and  marketing  bullion  were  2302.60  and 
40.3595,  respectively.  The  fine  ounces  in  460  Ib.  of  precipitate,  as  shown 
by  the  assays  at  the  smeltery  and  mill,  and  upon  which  basis  the  lot  of 
precipitate  was  sold,  were  2250.01  of  silver  and  39 . 857  of  gold.  The  gain 
in  favor  of  shipping  bullion  was,  silver,  52.59  fine  ounces;  gold,  0.5025 
fine  ounce. 

COMPARATIVE  COST  OF  SHIPPING  460  LB.  OF  PRECIPITATE  AND 
PRODUCING  AND  SHIPPING  THE  BULLION  FROM  460  LB.  OF 

PRECIPITATE 

Sampling  and  preparing  for  shipment $2. 16 

Drayage 0 . 55 

Express 15 . 94 

Sampling  at  smeltery 1 . 15 

Treatment  charges 22 . 55 

Deductions,  gold. 16 . 64 

Deductions,  silver , 25 . 86 


Total  cost  of  shipping  460  Ib.  precipitate $84.85 

Cost  of  melting  at  I0.011  per  oz.  dore  bullion $32.55 

.Canvas  on  bars,  etc .  : 0.30 

Drayage 0 . 48 

Express 1 1 . 86 

Refining  charges 33 . 38 


Total  cost  of  melting  460  Ib  precipitate  and  shipping 

as  bullion $78.57 

Net  amount  realized  shipping  bullion  was  $2074.04,  and  that  realized 
shipping  precipitate  was  $2026.88,  a  difference  in  favor  of  shipping 
bullion  of  $47.16. 

This  would  amount  to  a  saving  of  $0.02079  per  fine  oz.  of  dore  bullion 
(fine  ounces  gold  plus  fine  ounces  silver) . 

In  the  second  shipment  the  fine  ounces  of  silver  and  gold  realized  by 
melting  300  Ib.  of  precipitate  and  marketing  bullion  were  1297.71  oz. 
silver,  and  21 . 532  oz.  gold  respectively.  The  fine  ounces  in  300  Ib.  of 

1  This  figure  includes  the  cost  of  treating  the  byproducts.  It  is  unusually  high  on 
account  of  the  nature  of  the  precipitate. 


PRACTICE  AT  CRIPPLE  CREEK,  COLORADO  93 

precipitate,  as  shown  by  the  assays  at  the  smeltery  and  mill,  and  upon 
which  basis  the  lot  of  precipitate  was  sold,  were  of  silver,  1236.94  and  of 
gold,  21 . 61.  The  gain  in  favor  of  shipping  bullion  was  33.77  oz.  of  silver 
and  0.078  oz.  of  gold. 


COMPARATIVE  COST  OF  SHIPPING  300  LB.  OF  PRECIPITATE  AND 
PRODUCING  AND  SHIPPING  THE  BULLION  FROM  300  LB.  OF 

PRECIPITATE 

Sampling  and  preparing  for  shipment $1.41 

Drayage 0 . 36 

Express 11 . 14 

Sampling  at  smeltery 0 . 80 

Treatment  charges 12 . 83 

Deductions,  gold 9 . 08 

Deductions,  silver 14 . 66 


Total  cost  of  shipping  300  Ib.  precipitate $50.28 

Cost  of  melting  at  $0.01  per  oz.  dore  bullion $12. 11 

Canvas  on  bars,  etc 0.10 

Drayage 0.17 

Express 5 . 19 

Refining  charges 12 . 51 


Total  cost  of  melting  300  Ib.  precipitate  and  shipping 

as  bullion ,   $30.08 

The  net  amount  realized  when  shipping  bullion  was  $1167.66,  and  the 
net  amount  realized  when  shipping  precipitate  was  $1129.49,  a  difference 
in  favor  of  shipping  bullion  of  $38.17.  This  would  amount  to  a  saving  of 
$0.02969  per  fine  oz.  of  dore  bullion. 

The  saving  for  four  consecutive  months,  when  the  shipments  of  dore 
bullion  amounted  to  over  50,000  oz.  per  month,  was  in  July,  $1090.52; 
August,  $1056.21;  September,  $1178.67;  October,  $1003.61;  a  total  of 
$4329  saving  for  the  four  months. 

Although  the  cost  of  melting  (Ic.  per  ounce)  was  unusually  high  at  this 
plant,  due  to  the  refractory  nature  of  the  precipitate,  the  saving  in  favor  of 
local  melting  and  the  shipping  of  bullion  is  apparent.  I  have  obtained 
costs  of  melting  as  low  as  O.lc.  per  ounce  in  Central  America,  and  others 
have  done  much  better  than  this  where  a  large  volume  of  high-grade  silver 
precipitate  was  melted  under  the  most  favorable  conditions. 

Within  certain  limits  the  treatment  charge  upon  bullion  is  usually 
based  upon  a  flat  rate  per  gross  ounce,  so  that  while  apparently  the  rate 
is  no  higher  upon  bullion  containing  considerable  base  material,  as  a 


94  DETAILS  OF  CYANIDE  PRACTICE 

matter  of  fact  as  the  amount  of  base  material  increases,  the  rate  per  fine 
ounce  of  dore  also  increases  in  direct  proportion  to  the  increase  of  base. 
To  illustrate  how  this  might  affect  the  saving  due  to  melting,  I  have  cal- 
culated the  saving  which  would  have  resulted  if  bullion  950  fine  had  been 
shipped  instead  of  the  base  copper  bullion  from  the  plant  in  question. 
During  the  four  months  mentioned,  the  saving  in  July  would  have  been 
$1557.36,  that  in  August  $1508.40,  in  September  $1683.24,  and  in  Oc- 
tober $1433.25.  It  will,  therefore,  be  seen  that  where  a  high-grade  pre- 
cipitate is  produced  which  can  be  converted  into  comparatively  high-grade 
bullion  at  a  low  cost,  the  saving  by  local  melting  in  a  large  plant  is  greater 
than  that  indicated  by  the  figures  which  I  have  given. 

The  saving  indicated  in  this  case  is  plainly  due  to  two  causes :  First, 
that  the  ordinary  cyanide  plant  can  actually  convert  silver-gold  precipi- 
tate into  bullion  cheaper  than  the  charge  made  by  the  smelters  for  this 
service;  second,  that  with  local  melting  and  the  proper  treatment  of  the 
byproducts,  more  bullion  is  realized  than  that  indicated  by  the  ordinary 
uncorrected  assays. 

It  is  only  fair  to  say  that  these  comparisons  were  made  a  number  of 
years  ago  and  that  the  rates  of  treatment  for  precipitate  and  bullion  taken 
are  those  obtaining  at  that  time. 

G.  H.  CLEVENGER. 

Palo  Alto,  Calif.,  Mar.  16,  1913. 


In  the  discussion  regarding  the  shipment  of  cyanide  precipitate,  I 
would  like  to  direct  attention  to  one  reason  which  may  account,  in  some 
cases,  for  low  results  when  shipping  bars.  This  is  the  shipment  of  imper- 
fectly refined  bars,  which  yield  such  erratic  assay  results  that  the  pur- 
chaser must  necessarily  put  a  low  valuation  on  them  for  his  own  protec- 
tion. In  the  annual  report  of  the  British  Mint  for  1896,  Roberts-Austen 
describes  a  cyanide  bar  weighing  393  oz.,  which  was  paid  for  at  £965. 
This  bar  was  refined  by  itself  and  yielded  gold  worth  £1028.  The 
shipper  of  the  bar  lost  £63  or  over  $300.  After  paying  for  the  refining, 
such  a  loss  might  easily  be  fatal  to  shipment  of  bars  as  against  precipitate 
in  a  comparison  of  costs. 

In  the  JOURNAL  for  Apr.  13,  1912, 1  have  given.a  table  of  erratic  assays 
of  cyanide  bars  from  the  Mercur  mine.  It  would  be  a  hazardous  under- 
taking to  determine  from  these  assays  just  how  much  gold  these  bars 
contained.  Some  bullion  deposited  by  the  Mercur  mine  just  before 
closing  down  was  better  refined  and  higher  grade.  Samples  from  three 
bars  of  these  later  shipments  were  assayed  in  duplicate  in  three  labora- 
tories with  the  following  results: 


PRACTICE  AT  CRIPPLE  CREEK,  COLORADO 


95 


. 

1st  Bar  gold 

2d  Bar  gold 

3d  Bar  gold 

1st  Laboratory 

920  1  fine 

916  3  fine 

884.2  fine 

2d  Laboratory  
3d  Laboratory  

.2  fine 
920.8  fine 
.8  fine 
920.1  fine 
.3  fine 

.7  fine 
916.8  fine 
.5  fine 
916.5  fine 
.6  fine 

.^fine 
884.5  fine 
885.     fine 
884.0  fine 
.3  fine 

These  results  show  a  vast  improvement  over  the  former  assays  of 
Mercur  bars. 

FKEDERIC  P.  DEWEY, 

Assay  er,  Mint  Bureau. 
Washington,  D.  C.,  Apr.  28,  1913. 


CHAPTER  IX 
CONTINUOUS  DECANTATION  OF  SLIME 

The  advent  of  a  new  process  is  not  regarded  with  excitement  in  the 
metallurgical  world.  Too  many  of  them  have  appeared,  led  a  struggling 
existence  for  a  few  months,  or  longer  in  some  cases,  and  then  disappeared 
without  leaving  many  ripples  in  the  places  where  they  existed.  Develop- 
ment of  existing  and  proved  processes,  however,  is  regarded  with  interest, 
and  as  each  step  in  advance  proves  its  worth  it  is  incorporated  into  the 
system  of  approved  and  satisfactory  methods. 

In  the  days  when  the  decantation  method  of  treating  slime  was  the 
only  one  known,  operators  made  serious  efforts  to  improve  it,  particularly 


OPHIR  MILL,  OPHIR,  COLO. 

to  find  some  way  of  avoiding  the  losses  which  occurred  in  discharging 
residues  containing  moisture  charged  with  both  cyanide  and  valuable 
metal  in  dissolved  form.  The  necessity  for  improvement  brought  forth 
the  slime  filters,  and  for  a  long  time  experiments  in  decantation  were 
discontinued.  After  many  years  of  filtering,  however,  the  suspicion 
remains  that  the  decantation  process  might  be  improved  so  as  to  compete 
with  filtration,  the  cost  of  operation  and  maintenance  of  existing  filtering 
apparatus  going  far  toward  strengthening  that  suspicion.  It  may  be  said 

96 


CONTINUOUS  DECANTATION  OF  SLIME  97 

that  the  cost  of  slime  filtration  varies  from  lOc.  to  maximum  figures  which 
would  be  regarded  with  suspicion  were  I  to  mention  them.  Including 
cost  of  operation,  maintenance  and  loss  in  dissolved  metal,  the  minimum 
figure  above  mentioned  is  extremely  low  and  I  doubt  whether  there  are 
any  number  of  plants  in  existence  attaining  it.  It  will  be  seen  that  there 
is,  then,  an  opportunity  for  improvement,  for  if  any  procedure  can  suc- 
cessfully reduce  these  costs  or  losses,  or  even  equal  them  without  the  use 
of  complicated  and  expensive  machinery,  it  will  stand  a  good  chance  for 
approval  and  success. 

Development  of  Proposed  System. — Lately  some  progress  has  been 
made  toward  developing  a  process  of  continuous  decantation,  which  is 
nothing  new,  but  a  development  of  the  methods  used  before  the  filters 
came  into  use.  In  the  old  intermittent  decantation  process  the  slime 
after  being  agitated  with  a  quantity  of  solution  was  allowed  to  settle,  the 
solution  decanted  off  and  a  new  bath  of  solution  put  on.  This  was  in 
turn  agitated,  allowed  to  settle  and  again  decanted,  the  process  being 
carried  on  as  many  times  as  was  thought  necessary.  The  efficiency  of 
fresh  solution  washes  was  recognized,  particularly  when  this  solution  was 
barren  or  freshly  made  up  or  regenerated,  but  the  difficulty  with  this 
system  was  the  enormous  quantities  of  solution  to  be  handled  in  addition 
to  the  impossibility  of  reducing  the  moisture  content  of  the  final  pulp  to 
such  a  point  that  too  much  dissolved  metal  and  valuable  cyanide  should 
not  be  thrown  away.  This  new  development  attempts  to  accomplish  a 
solution  of  both  difficulties  by  means  of  a*  continuous-decantation  and 
thickening  process  using  solutions  flowing  opposite  to  the  course  of  the 
pulp.  The  fact  that  this  method  has  seemed  feasible  to  a  number  of 
competent  metallurgists  and  is  actually  in  operation  in  a  number  of  in- 
stances makes  it  worthy  of  attention  and  in  this  paper  two  such  plants 
will  be  described. 

Milling  in  Cyanide  Solution. — The  plants  in  question  are  the  mills  of 
the  Ophir  Gold  Mines,  Milling  &  Power  Co.,  at  Ophir,  San  Miguel  County, 
Colorado,  and  of  the  Blue  Flag  Gold  Mining  Co.,  in  the  Cripple  Creek 
district.  Both  are  of  recent  construction,  and  while  operations  have  not 
been  carried  on  long  enough  to  provide  satisfactory  details  of  cost  and 
maintenance,  the  metallurgy  has  shown  sufficient  promise  of  success  to 
warrant  consideration. 

The  Ophir  mill  crushes  ore  with  stamps,  the  installation  including  20 
of  850  Ib.  each  with  6-in.  drop  and  104  drops  per  min.  From  the  stamps 
the  pulp  goes  to  a  Dorr  classifier,  where  the  slime  is  passed  to  a  Dorr  Thick- 
ener and  the  sand  reground  in  a  Hardinge  mill,  from  which  it  is  pumped 
back  to  the  classifier,  the  circuit  being  closed  and  only  solids  of  sufficient 
fineness  reaching  the  thickener. 

A  point  of  interest  is  that  the  overflow  solution  from  the  first  thickener 


98 


DETAILS  OF  CYANIDE  PRACTICE 


is  the  only  one  precipitated.  It  is  passed  through  a  clarifier  and  precipi- 
tated with  zinc  dust.  The  thickened  pulp  is  passed  to  two  agitation  tanks 
of  the  Dorr  type,  through  which  it  passes  in  series  and  where  it  is  calcu- 
lated that  the  extraction  is  practically  completed.  Leaving  the  agitators, 


Tank 


[               Stamps 

ISOWreKCNI.O\ 

\dOOT.5oI.P  A  J.O 

r  "*  

A 

ISOT.Wa-t-er 


No.4Dorr 
Thickener 
33x12' 

-O     /OI//.JO/.         |  

"    V 

No.SDorr^ 

Thickener 
33'xl2' 

-of--—$" 
/AU.  0.035 

tycrt.o.31 

PA.O.Zl 

^  \L>      $ 
\U.O.Q1 
KCN.0.62 
PA.QC2 

i, 

.740  T.SIime                              \I50  T.SIimeAU.  0.20  KCN.0.31. 

ISOT.Sol. 


ISOT.Sol.  AU.0.03SP.A.0.3TTai'S 


FLOW  SHEET  OF  OPHIR  MILL. 


the  pulp  is  delivered  to  the  first  of  a  series  of  four  thickeners,  through 
which  it  passes  with  successive  dilutions  until  it  is  finally  discharged  from 
the  last  one,  washed  practically  free  of  gold  in  solution. 


CONTINUOUS  DECANTATION  OF  SLIME  99 

The  overflow  solution  from  the  first  of  these  thickeners  is  returned  to 
the  mill  solution  tank,  where  it  is  used  in  milling  and  classifying,  the 
result  being  a  concentrated  solution  for  precipitation.  The  barren 
solution  from  precipitation  goes  to  dilute  the  inflow  into  the  second 
thickener,  the  overflow  from  which  goes  back  into  the  first.  The  accom- 
panying flow  sheet  shows  the  movement  of  pulp  and  solution  throughout 
the  mill.  It  will  be  noted  that  water  is  taken  into  circulation  at  the 
end  of  the  washing  period,  the  logical  point  of  entry  for  it,  and  prac- 
tically replaces  the  solution  which  comes  with  the  solids  to  that  point. 

Progress  of  Solutions. — It  will  also  be  noticed  that  the  progress  of 
solutions  is  contrary  to  the  direction  of  pulp  flow,  the  solutions  being 
constantly  enriched.  The  figures  given  in  the  flow  sheet  are  based  on  ore 
carrying  $5  in  gold  and  the  contents  of  solutions  are  given  in  pounds  of 
KCN  and  lime.  The  calculations  of  thickening  are  based  on  withdrawing 
a  pulp  from  each  thickener  which  consists  of  half  solids  and  half  solution. 
This  is  accomplished  in  practice,  in  fact  a  slightly  thicker  pulp  can  be  ob- 
tained, thus  increasing  the  efficiency  of  the  process.  It  has  been  in  opera- 
tion only  a  short  time  but  the  results  obtained  have  been  in  complete 
accordance  with  the  figures  shown.  It  will  be  seen  that  the  installation  is 
simple  and  inexpensive,  may  be  operated  by  a  minimum  amount  of  labor 
and  accomplishes  a  washing  of  pulp  and  removal  of  dissolved  metals  that 
is,  to  say  the  least,  comparable  with  good  filtration  work. 

The  lime  in  this  mill  is  added  at  the  bins  and  goes  through  the  circula- 
tion with  the  pulp.  The  consumption  amounts  to  about  0.9  Ib.  per  ton 
of  ore.  The  value  of  the  process  depends  a  great  deal  on  using  solutions 
of  low  cyanide  content  and  the  existence  of  a  plentiful  water  supply,  which 
conditions  are  satisfactorily  met  in  this  case.  The  flow  sheet  shows  all 
the  details  of  the  process  and  apparatus  and  no  further  explanation  is 
needed. 

The  Cripple  Creek  Installation. — The  Blue  Flag  mill  is  situated  be- 
tween Cripple  Creek  and  Victor  in  Colorado.  The  ore  averages  between 
$5  and  $6  and  is  to  a  great  extent  from  the  dumps.  The  coarse  ore  is 
dumped  on  to  a  steel  conveying  and  picking  belt  where  a  portion  of  the 
barren  rock  is  removed.  From  the  picking  belt  it  is  passed  through^  a 
12X18-in.  Dodge  crusher  and  through  a  Symons  disk  crusher  from  which 
it  emerges  reduced  to  about  f-in.  cubes.  The  finely  crushed  ore  is  then 
delivered  to  a  special  form  of  Chilean  mill  which  consists  of  a  concave  die 
ring  7  ft.  in  diameter,  over  which  convex  rollers  pass.  The  rollers  are 
small,  weighing  only  700  to  800  Ib.  each.  The  mill  makes  36  r.p.m.  and 
grinds  the  ore,  in  cyanide  solution  of  1J  per  ton  strength,  through  a  30- 
mesh  screen.  Solution  used  at  the  rate  of  about  five  to  one. 

The  pulp  as  it  comes  from  the  mills  is  delivered  to  the  series  of  agi- 
tating tanks  through  which  it  passes  continuously.  These  tanks  are  14 


100 


DETAILS  OF  CYANIDE  PRACTICE 


ft.  diameter  and  16  ft.  deep  having  cone  bottoms  at  an  angle  of  55°.     The 
agitation  is  pneumatic,  elevating  the  pulp  through  a  central  tube. 

This  point  in  the  process  is  open  to  criticism,  as  the  pulp  from  the 
mills  is  in  too  coarse  condition  for  economical  agitation,  to  say  nothing 
of  the  metallurgy  of  it.  If  it  is  true  that  finer  crushing  is  not  necessary, 
then  it  would  seem  reasonable  to  separate  the  coarse  portion  of  the  solids 
and  treat  it  separately  by  some  more  economical  means.  The  pulp  under- 
going agitation  has  the  following  approximate  grading  analysis:  On  30 
mesh,  0  to  0.5%;  on  40  mesh,  4  to  9%;  on  60  mesh,  22  to  24%;  on  80 
mesh,  35  to  36%;  on  120  mesh,  58  to  60%  and  through  120  mesh,  40  to 
42%.  The  grading  statement  is  cumulative.  Pulp  of  this  class  requires 
excessive  power  to  keep  it  in  circulation. 


•  Solution 

Overflow 

Clarifier 
Precipji-ai-ion 


Wear 
Overflow 


DIAGRAM  OF  CONTINUOUS  COUNTER-CURRENT  DECANTATION  AT  BLUE  FLAG  MILL. 

Continuous  Washing. — The  pulp  is  in  circulation  through  the  agita- 
tion tanks  for  about  nine  hours  and  the  extraction  in  them  amounts  to 
about  70%.  In  considering  the  extraction  results  it  must  be  remem- 
bered that  this  is  on  the  refractory  ore  of  the  Cripple  Creek  district. 
The  cyanide  consumed  in  the  process  is  about  0.35  Ib.  of  which  0.15  Ib. 
is  mechanical  loss.  Lime  is  added  to  the  ore  bins  and  progresses  with  the 
pulp,  the  consumption  being  about  2?  Ib.  per  ton  milled. 

The  pulp  from  the  agitators  is  taken  to  a  series  of  four  thickeners  and 
is  washed  through  them  after  the  manner  described  in  the  Ophir  mill. 
The  accompanying  diagram  shows  the  flow  of  pulp  and  solutions  through 
them.  In  the  case  of  the  Blue  Flag  mill,  also,  it  has  been  found  possible 
to  secure  a  thickened  pulp  containing  less  than  50%  moisture.  In  this 
instance,  also,  it  has  been  found  that  the  calculations,  based  on  50% 
thickening,  are  borne  out  in  practice. 

Other  Combinations. — It  should  be  remembered  that  this  development 
is  not  recommended  to  cure  all  the  ills  that  cyaniders  are  heir  to,  but 
is  a  progression  along  conservative  lines  and  one  that  should  eventually 
lead  to  a  fixed  system  available  in  cases  where  the  character  of  the  ore 


CONTINUOUS  DECANTATION  OF  SLIME  101 

will  bear  such  treatment  and  where  other  local  conditions  are  favorable. 
It  is  a  system  which,  while  not  yet  perfected,  will  be  well  worth  following. 
It  is  easy  to  see  that  other  combinations  are  possible,  such  as  using  a 
greater  number  of  thickeners,  of  this  type  or  any  other  satisfactory  one, 
and  by  precipitating  two  or  more  different  solutions.  The  two  installa- 
tions mentioned  are  not  by  any  means  the  only  ones  using  it.  They  may 
not  even  be  the  ones  where  it  is  most  advanced,  the  idea  in  presenting  the 
matter  being  principally  to  draw  attention  to  the  fact  that  there  is  a 
system  in  course  of  development  which  may  possibly  lead  to  decided 
economies  in  the  cases  of  ores  to  which  it  is  applicable. 


CHAPTER  X 
PRACTICE  AT  TONOPAH 

The  Tonopah  district  offers  to  the  student  of  contemporary  metal- 
lurgy what  is  probably  the  most  interesting  demonstration  of  the  efficiency 
of  the  cyanide  process  applied  to  silver  ores.  This  is  not  to  say  that  the 
general  methods  in  use  are  original,  as  they  are  not,  but  the  application 
of  these  methods  has  been  attuned  to  the  requirements  of  the  particular 
ores  to  be  treated  and  the  problems  have  been  attacked  and  solved  with 
praise  worthy  conservative  judgment.  It  is  both  interesting  and  note- 
worthy that  in  all  the  Tonopah  district  there  is  not  one  idle  reduction 
plant,  nor  one  which  has  been  designed  by  a  tyro  to  include  exaggerated 
ideas  or  " freaks."  All  the  plants  conform  to  standard  practice,  are 
built  after  tried  and  proved  plans  and  embody  only  such  variations  as 
have  seemed  best  adapted  to  care  for  the  characteristics  of  the  local  ores. 

It  is  also  noteworthy  that  there  is  not  a  mill  in  Tonopah  which  has  not 
paid  for  itself  out  of  profits  won  from  the  ores  of  the  mine  it  was  designed 
to  treat,  the  only  exception  at  the  present  time  being  one,  or  possibly  two, 
of  the  most  recently  built  mills  which  have  not  yet  had  time  to  accomplish 
this  feat,  but  which  will,  without  any  question,  complete  the  requirements 
within  a  few  months.  Possibly  the  most  noteworthy  accomplishment  of 
this  kind  is  that  of  the  West  End  mill,  which  returned  sufficient  profit 
within  seven  months  to  pay  for  the  installation.  It  is  to  be  remembered 
that  this  is  an  old  mill,  rebuilt  twice  before  reaching  its  present  condition, 
but  still  performing  admirable  work  at  a  cost  which  is  comparable  with 
that  attained  by  the  more  recent  plants. 

The  Ores  of  Tonopah. — The  geology  and  character  of  the  Tonopah 
ores  have  been  sufficiently  described  in  the  technical  press  from  time  to 
time  and  it  will  be  sufficient  to  consider  them  here  only  in  Delation  to 
their  attitude  toward  cyanidation.  They  may  be  said  to  be  of  medium 
hardness  and  crush  without  any  great  amount  of  trouble.  This  statement 
is  made  not  so  much  in  considering  the  ores  as  a  single  problem,  but  as 
comparing  them  with  the  rock  treated  in  the  cyanide  plants  of  other 
districts.  For  instance,  it  might  be  said  that  the  Tonopah  ores  are  some- 
what harder  than  that  of  the  Porcupine  district,  which  differ  essentially, 
of  course,  in  character,  and  not  so  hard  as  many  of  those  of  the  Cobalt 
district.  They  are  perhaps  about  equal  to  most  of  the  ores  of  the  Pozos  dis- 
trict of  Guanajuato  and  some  of  those  of  the  Guanajuato  and  Pachuca 

102 


PRACTICE  AT  TONOPAH  103 

districts  of  Mexico.  They  may,  briefly,  be  said  to  be  medium  ores  as 
far  as  resistance  to  fine  reduction  is  concerned. 

There  are  under  treatment,  as  is  usual  in  districts  as  comparatively 
recent  as  Tonopah,  both  oxidized  and  sulphide  ores.  The  oxidized  ores 
have  not  presented  any  great  difficulties  of  metallurgy,  but  the  sulphides 
have,  in  many  cases,  presented  a  complexity  of  nature  which  is  not  so 
simply  handled.  Reference  to  the  current  descriptions  of  these  ores  will 
show  their  character,  but  it  is  probably  not  superfluous  to  call  attention 
to  the  fact  that  in  some  of  the  high-grade  sulphide  ores  mercury  has  been 
found  in  small  quantities.  I  have  seen  a  specimen  of  sulphide  ore  which 
shows  a  quantity  of  cinnabar.  These  sulphides  are  not  always  simply 
treated  and  in  some  cases,  as  will  be  noted,  it  was  found  advisable  to 
remove  them  by  concentration  before  proceeding  to  the  cyanidation  of 
the  bulk  of  the  ore. 

The  mill  of  the  West  End  Consolidated  Mining  Co.  is  situated  at  the 
southeast  edge  of  the  town  of  Tonopah.  At  the  mine  shaft  the  ore  is 
crushed  to  break  up  all  large  boulders.  The  ore  is  brought  to  the  mill  in 
auto  trucks  and  delivered  into  a  bin,  from  which  it  passes  through  a 
10X20-in.  Blake  crusher.  A  bucket  elevator  delivers  the  crushed  ore 
into  a  32X72-in.  trommel  with  1-in.  apertures,  the  oversize  going  to  a 
No.  3  Kennedy  crusher.  The  crusher  product  joins  the  undersize  from 
the  trommel  and  is  carried  by  a  belt  conveyor  with  14°  incline,  14-in. 
belt,  to  the  200-ton  battery  bin. 

Stamp  Installation.— There  are  10  stamps  of  1200  Ib.  and  10  of  1300 
lb.,  fed  by  means  of  suspended  Challenge  feeders.  The  stamps  drop 
through  6J  in.  101  times  per  minute.  The  stamp  duty  at  present  is 
about  7.5  tons  per  day  per  stamp. 

From  the  stamps  the  pulp  is  fed  into  two  Dorr  classifiers,  which  deliver 
into  two  5  X  18-ft.  tube  mills.  The  classifiers  are  placed  close  in  front  of 
the  batteries,  and  the  tube  mills  are  ranged  with  their  long  axes  parallel 
to  the  batteries,  the  discharge  end  of  one  of  them  fronting  the  feed  end 
of  the  other.  This  has  been  done  with  the  idea  of  occupying  the  least 
possible  space  in  the  direction  of  the  mill  fall.  As  the  mill  is  a  recon- 
structed one,  having  formerly  been  the  first  amalgamating  mill  in  the 
district,  a  free  hand  in  designing  the  arrangement  was  not  obtainable, 
but  the  advantages  have  been  well  handled  and  the  arrangement  is 
such  as  to  permit  economical  handling  of  the  material. 

Tube-Mill  Linings. — It  will  be  noticed  that  the  tube  mills  in  use  are  of 
large  diameter  and  short  length.  It  seems  that  this  type  is  meeting  with 
almost  universal  approval  from  metallurgists  of  the  present  day,  the  con- 
clusion being  that  in  the  former  long  mills  there  was  a  great  deal  of  space 
at  the  discharge  end  which  did  not  perform  any  useful  work.  The  change 
toward  larger  diameters  and  less  length  has  resulted  in  an  increased  pro- 


104 


DETAILS  OF  CYANIDE  PRACTICE 


PRACTICE  AT  TONOPAH  105 

duction  of  slime  at  a  power  consumption,  per  ton  of  ore,  equal  to  or  less 
than  with  the  long  tubes.  There  is  an  added  advantage  in  the  smaller 
floor  space  occupied. 

At  the  West  End  mill  the  linings  used  in  the  tube  mills  are  of  the 
smooth,  cast-iron  type,  as  hard  as  they  can  be  made.  The  operators 
claim  that  with  this  lining  the  power  consumption  is  less  than  when  silex 
is  used  or  any  of  the  ribbed  types  and  that  the  output  is  satisfactory  in 
quantity  and  character.  In  this  point  the  investigator  has  to  face  a 
peculiar  problem  which  seems  to  have  no  direct  solution.  Smooth  linings 
for  tube  mills  were  used  in  the  days  long  ago  and  were  discarded  in  most 
places  for  silex  or  the  ribbed  types,  where  the  wear  does  not  come  directly 
upon  the  lining,  or  at  any  rate,  is  not  supposed  to  do  so.  Yet  we  have 
here  a  recurrence  to  the  older  type,  with  the  claim  of  better  results.  This 
circumstance  would  not  be  so  strange  if  the  operators  in  the  district 
working  upon  the  same  character  of  ore  were  in  accord,  but  they  are  not. 
The  Belmont  mill  has  made  tests  using  silex  in  competition  with  smooth 
linings  and  giving  preference  to  the  silex.  Tests  have  also  been  made  at 
the  West  End  mill,  and  the  result  is  said  to  be  in  favor  of  the  smooth 
lining. 

In  cases  of  this  kind  no  definite  conclusions  can  be  drawn  at  the 
present  time,  the  only  recourse  being  to  await  developments,  it  being 
more  than  probable  than  the  operators  in  the  district  will  eventually 
agree  upon  the  most  effective  form.  It  is  noteworthy  that  both  the 
smooth  cast-iron  linings  and  the  silex  linings  were  tried  out  in  Mexico 
years  ago  and  discarded  in  favor  of  ribbed  linings.  The  Komata  form 
was  tested  here,  but  the  increased  power  required,  together  with  its 
high  cost,  have  prevented  its  use.  At  the  Goldfield  Consolidated  mill, 
however,  these  linings  are  in  use  and  it  is  stated  that  they  are  the  most 
economical  and  effective  ever  tried.  It  is  true  that  there  is  an  increase 
in  the  power  required  to  drive  the  mill,  but  the  increased  production 
more  than  compensates  for  the  additional  power  used. 

When  the  West  End  plant  was  first  started  both  of  the  tube  mills  were 
moved  by  a  100-hp.  motor  and  the  pebble  charge  was  carried  at  or  slightly 
below  the  center  of  the  mill.  The  power  was  ample  with  this  arrange- 
ment. With  the  desire  to  increase  capacity,  experiments  were  made  with 
the  object  of  securing  greater  capacity  by  increasing  the  charge  of  pebbles 
in  the  tube  mills.  It  was  soon  found  that  the  power  was  insufficient,  but 
as  a  result  of  many  tests,  the  100-hp.  motor  formerly  driving  the  two  mills 
was  replaced  by  a  150-hp.  motor  and  the  pebble  charge  carried  six  inches 
above  the  mill  center. 

Solely  by  this  change  the  capacity  of  the  plant  was  increased  from  100 
to  150  tons  per  day,  the  increased  work  performed  by  the  tube  mills 
allowing  a  coarser  screen  to  be  used  on  the  batteries.  The  actual  power 


106  DETAILS  OF  CYANIDE  PRACTICE 

taken  by  the  two  mills  is  from  120  to  125  hp.  This  result  is  noteworthy* 
and  has  the  effect  of  rendering  the  use  of  the  counter-spiral  discharge  un- 
suitable for  a  mill  of  the  kind,  as  the  high  pebble  charge  is  likely  to  force 
out  some  of  the  pebbles  through  the  discharge  end.  A  grating  is  usually 
used  in  mills  of  the  kind,  which,  of  course,  precludes  the  feeding  of  pebbles 
at  the  discharge  end.  Experiments  with  tube  mills  of  large  diameter  and 
varying  pebble  charges  ought  to  be  productive  of  interesting  results. 

Concentrating  the  Slime. — The  tube-mill  product  is  returned  to  the 
classifiers  by  means  of  a  bucket  elevator  and  the  slime  from  the  classifier? 
is  taken  to  the  concentrating  department  of  the  mill  which  is  of  recent  in- 
stallation. The  slime  is  led  to  four  8-ft.  Callow  thickening  cones,  the 
thickened  slime  being  concentrated  on  12  Deister  slime  concentrators. 
After  concentration  the  slime  tailing  from  the  concentrators  is  returned  to 
four  Dorr  thickeners,  16X10  ft.  each,  part  of  the  overflow  being  used  for 
the  concentrator-table  washes  and  returning  with  the  pulp  to  the  thick- 
eners. The  slime  pulp  contains  about  90%  through  a  200-mesh  screen. 

Four  diaphragm  pumps  are  used  for  lifting  the  thickened  slime  from 
the  thickeners  into  the  agitation  tanks.  The  suction  of  these  pumps  is 
connected  directly  to  the  outlet  from  the  thickeners  and  consequently  any 
regulation  of  thickener  discharge  has  to  be  made  by  varying  the  adjust- 
ment of  the  pumps.  Personal  experience  has  not  been  such  as  to  inspire 
a  great  deal  of  confidence  in  diaphragm  pumps,  as  they  have  usually 
proved  unreliable  and  likely  to  lose  their  priming  or  break  the  diaphragms 
at  most  inopportune  times,  but  at  the  West  End  mill  they  are  working 
successfully  and  have  given  entire  satisfaction.  Due  to  the  fact  that  they 
are  essentially  a  suction  pump,  they  have  been  placed  at  a  height  where 
their  delivery  can  flow  by  gravity  into  the  agitation  tanks. 

The  system  of  having  the  pump  suction  connected  directly  to  the 
thickener  has  the  advantage  of  avoiding  spilling  of  the  pulp  and  makes  a 
direct  and  tight  connection  from  the  thickeners  to  the  agitation  tanks,  but 
the  disadvantage  is  that  adjustment  or  change  of  the  thickener  discharge 
has  to  be  made  on  the  pumps,  either  by  changing  the  throw  or  speed, 
making  the  operation  slow  and  necessarily  performed  at  a  distance  from 
the  thickener  itself.  It  seems  possible  that  a  preferable  arrangement 
would  be  to  allow  the  thickened  slime  to  fall  directly  into  an  appropriate 
sump  from  which  it  could  be  picked  up  and  delivered  to  the  elevation  re- 
quired. This  change  would  allow  of  prompt  and  effective  regulation  of 
the  thickener  outflow. 

Agitation  System. — There  are  four  agitation  tanks,  each  24X8  ft., 
arranged  for  continuous  agitation  and  carrying  Trent  agitators.  The 
continuous  arrangement  is  such  that  in  the  passage  from  tank  to  tank  the 
pulp  is  required  to  pass  through  the  pump  suction  and  enter  each  tank 
through  the  Trent  agitators,  thus  insuring  the  maximum  agitation  and 


PRACTICE  AT  TONOPAH 


107 


avoiding  short  circuits  across  the  top  of  the  tank.  The  centrifugal  pumps 
used  are  the  Kelly  &  Campbell,  a  local  make  which  has  been  designed  to 
make  replacement  of  worn  parts  easy  and  rapid,  and  the  wearing  part  or 

200  Ton  R.R.  Bin 
1-10x20 "Blake  Crusher 
Be  It  a  net  Bucket  Elevator 

l-32x iz"jrommel,  /"Apertures, Undersize l'/4n 


Battery 
Solution 
Storage 


u 


J 


J-No.3  Kennedy  Crusher 
I- 14"  Be  It  Conveyor,  Id"  Incline 

200-ton  Battery  Bin 

4- Suspended  Challenge  Feeders 

4-5  Stamp  Batteries  (2-!200/b.  2-l300lb\l°l'nDr0P5 

/  Qp-  Drop 

2~ Dorr  Duplex  Classifiers 

\   2-5x18  Tube  Mills 

\ 

> 

h  Bettand  Bucket  Elevator 

4-8' CaKow  Cones 

12- Dei sfer Slime  Concentrators 

4-l6xIO'Dorr  Thickeners 

4~  Ho.Z  Could  Diaphragm  Pumps 

4-24x8'  Trent  Ag  if atorTanh,  4"Centrifugal 

Pumps 
l-28'x22'  Dorr  Thickener 

1-8x28'  Stock  Tank 

I.-IOO  Leaf  Butters  Filter,  5%"x8"Cycloidal 

Vacuum  Pump 
Slime  to  Waste  ^ 


o 


Solution 


Pregnant  Solution  Tank 
l-Ba-ltery Solution  Sump 
Pregnant  Solution  Storage 
6-6'Compartnienf  Zinc  Boxes 


I- Dry  ing  Pan 
I -Fa  her  du  Faur  fie  If  ing  Furnace 

\Bullion 


SlagtoSmeltei 

FLOW  SHEET  OF  WEST  END  MILL. 


liner  is  designed  to  require  no  machining  so  that  it  can  be  chilled  all  over. 
These  pumps  are  used  throughout  the  mill  and,  in  fact,  throughout  the 
district,  and  have  given  entire  satisfaction. 


108  DETAILS  OF  CYANIDE  PRACTICE 

The  difficulties  encountered  with  Trent  agitators  at  other  mills, 
notably  the  Hollinger,  at  Porcupine,  Canada,  and  at  the  Mexican,  in 
Virginia  City,  Nev.,  are  not  found  at  the  West  End  mill,  nor  in  some  other 
mills  in  the  district  where  they  are  largely  used,  as  will  be  noted.  Here 
they  give  satisfactory  results  and  the  operators  are  well  pleased  with  their 
performance.  The  centrifugal  pumps  used  have  4-in.  delivery  and  give  a 
pressure  of  from  14  to  16  Ib.  per  sq.  in.  The  agitator  arms  make  3J 
r.p.m.  and  the  operation  of  the  agitator  is  said  to  require  from  7  to  9  hp., 
varying,  naturally,  with  the  consistency  of  the  pulp  and  somewhat  on 
other  temporary  variations. 

From  the  agitation  tanks  the  pulp  is  elevated  by  means  of  a  6- in. 
centrifugal  pump  of  the  type  already  mentioned  into  a  28X22-ft. 
Dorr  thickener,  the  overflow  solution  going  to  the  pregnant-solution 
tank  and  the  thickened  slime  into  an  8X28-ft.  stock  tank,  which  feeds 
the  filter. 

The  filter  is  a  100-leaf  installation  of  the  usual  stationary  vacuum  type 
and  presents  no  novelties  in  the  system  of  operation.  The  slime  is  of 
such  character  that  a  light,  soft,  spongy  cake  can  be  made  which  is  easy 
to  wash,  a  cake  1J  in.  thick  being  readily  made  and  washed.  As 
proof  of  the  ready  washing  character  of  the  material  the  operators  have 
shown  that  shortly  after  one  replacement  of  the  contained  solution  is 
made,  the  value  of  the  effluent  solution  drops  down  nearly  to  the  value  of 
the  wash  solution  used.  By  a  "  replacement "  is  meant  the  passing 
through  the  cake  of  a  quantity  of  solution  equal  to  the  amount  contained 
as  moisture  in  the  cake.  By  passing  another  replacement  of  water 
through  the  cake  the  cyanide-bearing  solution  is  nearly  entirely  displaced, 
the  water  taking  its  place.  It  should  be  remembered,  however,  that  a 
cake  of  this  homogeneity  and  porosity  is  unusual  and  it  would  be  difficult 
to  point  to  many  examples  of  the  kind.  A  IJ-in.  cake  is  made  in 
from  45  to  60  minutes  and  is  given  a  three-hour  solution  wash  in  order  to 
take  advantage  of  the  additional  extraction  noted  below.  Due  to  the 
local  conditions,  the  residue  from  the  filtering  operation  has  to  be  repulped 
with  a  minimum  of  water  and  pumped  away,  there  being  no  available 
method  of  running  to  waste  by  gravity. 

Rotary  Vacuum  Pump. — The  vacuum  pump  is  a  type  I  have  not  seen 
used  for  this  purpose,  and  as  it  has  advantages  which  are  material,  it  is 
well  worth  mention.  It  is  a  Connersville  cycloidal  machine,  5JX8  in., 
and  has  2j-in.  suction  and  discharge.  It  consumes  only  three  to  four 
horsepower  and  maintains  a  vacuum  of  19  in.  at  this  altitude  (6000  ft.) 
The  machine  is  low  in  first  cost,  occupies  an  exceedingly  small  floor  space 
and  has  an  admirable  maintenance  cost,  there  having  been  no  repairs  on 
it  since  its  installation.  Machines  of  this  class  ought  to  be  investigated 
for  this  work,  as  the  record  of  this  particular  installation  is  admirable 


PRACTICE  AT  TONOPAH  109 

in  the  extreme.     It  is  constructed  like  a  positive  blower,  having  two  Jobes 
revolving  in  conjunction  with  each  other. 

The  filter  tank  is  equipped  with  air  lifts  to  aid  in  keeping  the  material 
in  suspension.  After  forming  the  cake  on  the  filter  it  is  given  a  solution 
wash  for  three  hours,  during  which  time  an  additional  extraction  of  silver 
of  from  1  to  2%  is  gained.  The  point  of  additional  extraction  is  extremely 
interesting  and  is  one  which  has  been  verified  in  several  of  the  mills  in 
the  district,  the  additional  metal  dissolved  varying  from  1  to  3%  in  vari- 
ous plants. 

Of  the  filter  leaves,  five  are  acid-treated  each  day,  using  HC1  of  0.5 
to  1  %  strength.  It  has  been  shown  in  many  mills  of  late  that  the  fre- 
quent treatment  of  filter  leaves  in  dilute  acid  lengthens  their  life  con- 
siderably and  does  not  tend  to  harden  the  canvas  as  does  more  concen- 
trated acid. 

Zinc  shavings  are  used  in  the  usual  way  for  precipitating  the  metal 
from  solutions,  the  installation  consisting  of  six  boxes  of  six  compart- 
ments each.  The  precipitate  is  collected,  partially  dried  and  melted  in 
Faber  du  Faur  furnaces,  which  are  equipped  with  a  special  low-pressure 
burner. 

The  ore  treated  varies  in  value  from  $18  to  $20  per  ton,  the  propor- 
tion of  silver  to  gold  being  about  as  100:1.  The  extraction  in  the  mill 
varies  from  90  to  92%  of  the  contained  metal.  The  power  consumed  is 
two  horsepower  per  ton  of  ore  treated,  and  the  regular  force  of  operatives 
consists  of  18  men. 

The  consumption  of  the  principal  supplies  used  in  cyaniding  is  as 
follows:  KCN,  3  Ib.  per  ton  of  ore  treated;  CaO,  3  lb.;  lead  acetate, 
0.5  lb;  zinc,  1.8  lb.;  pebbles,  6  lb.  The  total  cost  of  treatment  amounts 
to  about  $3.30  per  ton  of  ore  treated.  The  accompanying  flow  sheet 
gives  the  details  of  the  mill  operation  and  the  machinery  used. 

The  Montana-Tonopah  Mill. — The  Montana-Tonopah  mill  is  one  of 
the  original  mills  in  the  district  to  treat  ores  by  total  sliming.  It  was 
constructed  to  make  use  of  concentration,  but  this  has  been  partially 
eliminated  and  now  is  using  only  a  partial  concentration  system. 

The  ore  is  first  put  through  a  No.  5K  Gates* gyratory  crusher,  from 
which  it  is  elevated  and  passed  through  two  No.  3D  short-head  crushers 
of  the  same  make,  thence  going  by  means  of  a  14-in.  belt  elevator  into 
the  battery  bin. 

There  are  40  stamps  of  1100  lb.,  dropping  102  times  per  minute 
through  7J  in.  The  screen  used  is  a  " ton-cap"  equal  to  about  25 
mesh.  Stamp  crushing  is  universal  in  the  camp  and  the  Montana- 
Tonopah  is  in  line  with  general  practice. 

From  the  batteries  the  pulp  is  classified  in  cone  classifiers,  the  slime 
going  to  the  settlers  and  the  sand  being  concentrated  on  eight  Wilfley 


110 


DETAILS  OF  CYANIDE  PRACTICE 


PRACTICE  AT  TONOPAH  111 

sand  tables.  The  sand  from  the  concentrators  passes  to  two  Dorr  duplex 
classifiers  operating  in  closed  circuit  with  two  Allis-Chalmers  tube  mills, 
each  5  X  22  ft.  The  grinding  capacity  of  this  mill  is  in  excess  of  its  leach- 
ing and  agitating  capacity  and  only  one  of  the  tube-mill  sets  is  in  use,  the 
other  being  held  in  reserve  so  that  no  time  is  lost  on  account  of  relining 
or  repairing  the  tube  mill. 

It  will  be  noticed  that  the  tube  mills  in  this  installation  are  the  long 
type,  which  is  due  to  the  fact  that  they  were  installed  before  the  general 
opinion  turned  toward  the  short,  large-diameter  mills.  Most  of  the  tube 
mills  used  at  present  in  the  district  are  the  latter  type,  which  is  most  gen- 
erally favored.  Smooth  cast-steel  linings  are  used  in  the  tube  mills,  this 
arrangement  having  proved  satisfactory  in  efficiency  and  economy. 

Types  of  Agitators  in  Use. — The  slime  is  settled  in  cone-bottom  tanks, 
of  which  there  are  four,  each  30X10  ft.,  with  a  9°  pointed  bottom.  The 
thickened  slime  is  taken  to  one  of  eight  agitation  tanks,  which  are  operated 
on  the  intermittent-charge  system.  There  are  six  22X15-ft.  tanks,  con- 
taining Hendryx  agitators;  one  22X12-ft.  tank,  and  one  27Xl2-ft.  tank, 
each  equipped  with  a  Trent  agitator. 

In  this  plant  an  opportunity  has  been  had  to  compare  the  work  and  re- 
sults accomplished  with  the  Trent  and  Hendryx  systems,  and  it  is  note- 
worthy that  the  operators  prefer  the  Hendryx,  although  admitting  that 
it  requires  a  greater  amount  of  power  to  drive  it,  resulting  in  a  greater 
expense.  It  is  claimed  that  the  extraction  results  are  better  with  the 
Hendryx  device  and  that  it  gives  a  great  deal  less  trouble  than  the  Trent. 
The  main  difficulty  with  the  latter  seems  to  be  that  it  is  extremely  likely 
to  become  stopped  up  with  splinters  or  caked  slime  and  also  that  it  is  not 
so  efficient  in  getting  extraction  results.  This  matter  of  difference  of 
opinion  in  reference  to  agitators  will  be  mentioned  again  in  discussing 
further  the  practice  in  this  district. 

The  agitation  in  these  tanks  is  continued  for  54  hr.  in  a  solution  con- 
taining 5  Ib.  KCN  per  ton.  Lime  is  added  to  the  agitators  in  powdered 
form,  the  consumption  being  three  pounds  per  ton  of  ore.  The  treatment 
solutions  are  kept  heated  to  110°,  by  which  means  the  extraction  of  silver 
is  said  to  be  materially  increased. 

Stationary  Vacuum  Filter. — The  filter  installation  consists  of  a  sta- 
tionary plant  of  the  Butters  type,  containing  a  total  of  140  leaves,  of 
which  only  100  are  required  to  handle  the  capacity  of  the  mill.  The  filter, 
is  worked  in  five  cycles  per  24  hr.,  three  hours  being  used  for  taking  out 
and  acid-treating  the  leaves,  the  filtering  operation  ceasing  for  that  period. 
Each  day  six  leaves  are  treated,  and  weak  acid  is  used  for  the  process,  the 
average  being  about  2%  HC1.  Frequent  treatment  with  weak  acid  keeps 
the  filter  leaves  in  good  condition,  avoids  accumulation  of  large  quantities 
of  lime  on  the  canvas  and  notably  lengthens  the  life  of  the  leaf.  The  weak 


112 


DETAILS  OF  CYANIDE  PRACTICE 


acid  does  not  destroy  the  thread  used  in  sewing  the  leaf  and  does  not 
harden  the  canvas  as  strong  acid  does.  In  treating  the  leaves  frequently, 
only  a  small  quantity  of  lime  is  allowed  to  accumulate  on  the  leaves  and 
the  weak  acid  is  effective  in  removing  it. 

(©)      l-Ho.5K.6ates  Crusher 

Q       Elevator 
(°f      ©  2-Ho.3D.Shorthead.  6ates  Crushers 


Battery 
Storage  Tank 


1-14 '  Belt  Elevatot 


_J   Battery  Bin 

40-llOOIb.  Stamps,  102 Drops-  7j  Drop.Challenge 
25  flesh  Ion-cap  Screen  feeders 

8~Cone  Classifiers 

8-  Wifley  Sand  Concentrators 

2~  Duplex  Dorr  Classifiers 
•^ 

^  '2-5x22  Allis-Chalmers  tube  mills, 
2"  Smooth  Chi  I  led  Liners 


4  -  30x10  'done-bottom  Settlers, 


6-/sJ(22rHendryx  Agitators^  Intermittent 
I  -12x22'  Trent  Agitators       \  Charge 
I-  12x27'  Trent  Agitators      I  System 


1-500  ton  Stock  Tank 
1-100  Leaf  Butters  Fitter 


Slime  to  Waste 

Pregnant  Solution  Tank 
I- Zinc-dust  Feeder 

2- Merrill  P.recipitate  Presses 
l~SumpTank 


l~0il  Fired  Furnace 
FLOW  SHEET  OF  MONTANA-TONOPAH  MILL. 

The  time  of  forming  the  cake  is  45  min.,  a  IJ-in.  cake  being  built  up 
in  this  time.  Solution  wash  is  continued  for  three  hours  and  a  slight  addi- 
tional extraction  is  gained  in  this  time.  The  additional  extraction  gained 
on  the  filter  is  so  small  in  this  case  as  to  be  almost  negligible,  thus  differing 
from  the  experience  of  some  plants  in  the  district,  where  appreciable 
quantities  of  metal  are  dissolved  during  the  solution-washing  period. 


PRACTICE  AT  TONOPAH  113 

Due  to  the  fact  that  a  railroad  spur  runs  through  the  property,  it  was 
necessary  to  design  the  mill  in  two  units,  one  of  which  includes  the  crush- 
ing, grinding  and  concentrating  units  and  the  other  the  cyanide  plant. 
This  disadvantage  is  somewhat  offset  by  the  advantage  gained  in  receiv- 
ing supplies  directly  at  the  mill  and  the  loading  of  concentrate  without 
haulage  expense.  The  concentrate  is  shipped  to  a  smelting  plant. 

Zinc -dust  Precipitation. — The  Merrill  system  of  zinc-dust  precipitation 
is  used,  the  consumption  of  zinc  being  equal  weight  with  the  bullion  re- 
covered. The  precipitate  is  melted  in  oil-fired  furnaces,  no  acid  treat- 
ment being  practised. 

The  total  extraction  resulting  from  the  ore  treatment  is  about  92.6% 
and  this  total  is  divided  into  20%  in  concentration,  20%  in  contact  with 
solutions  and  52.6%  in  agitation.  The  cost  of  benefication  is  placed  at 
$3.09,  which  figure  does  not  include  the  general  expense  items.  The  con- 
sumption of  material  is  8  Ib.  lime  per  ton  of  ore  treated,  3  Ib.  cyanide, 
0.75  Ib.  lead  acetate;  pebbles,  1J  Ib.  per  ton  treated.  The  mill  treats 
from  145  to  150  tons  per  day,  and  the  ore  contains  20  oz.  silver  and  about 
$4  gold.  The  mill  requires  320  hp.  for  its  operation  and  employs  19  men 
regularly. 

As  this  is  the  oldest  mill  in  the  district  maintaining  its  original  con- 
struction and  treatment  plan,  some  of  the  later  ideas  are  not  included. 
This  is  to  be  especially  noted  in  the  case  of  the  tube  mills,  the  long  type 
being  used  instead  of  the  shorter  ones,  which  conform  to  later  ideas. 


CHAPTER  XI 
PRACTICE  AT  TONOPOH— Continued 

The  mill  of  the  Tonopah  Extension  Mining  Co.  is  situated  at  the 
edge  of  the  town  of  Tonopah.  It  is  one  of  the  later  mills  and  embodies 
more  recent  ideas  in  metallurgy  of  the  Tonopah  ores.  Some  of  these 
departures  are  of  much  interest,  among  them  the  use  of  tube  mills  in 
tandem. 

The  ore  is  first  crushed  through  a  No.  4  Kennedy  gyratory  crusher  and 
is  then  taken  by  an  inclined  belt  conveyor  to  the  mill  bins.  From  these 
it  is  fed  through  suspended  Challenge  feeders  into  the  stamps.  There 
are  thirty  1050-lb.  stamps  having  7-in.  drop  and  100  drops  per  min. 
Four  of  the  batteries  are  equipped  with  12-mesh  screens  and  two  with 
3-mesh  screens  of  the  "ton-cap"  type,  the  object  of  the  use  of  these 
different  screens  being  to  furnish  a  pulp  of  varied  coarseness  with  the  idea 
of  increasing  the  efficiency  of  the  tube  mills.  It  is  maintained  that  the 
coarser  portion  of  the  pulp  not  only  provides  a  means  of  satisfactorily 
using  a  part  of  the  grinding  action  of  the  pebbles  which  is  expended  in 
crushing  upon  themselves,  but  actually  assists  in  grinding  the  finer 
portions  into  slime. 

Tube-mill  Arrangement. — From  the  batteries  the  pulp  is  received 
by  a  duplex  Dorr  classifier,  which  removes  the  slime  and  delivers  the 
sand  and  coarser  rock  into  the  first  tube  mill.  This  is  a  machine  5  ft. 
diameter  and  18  ft.  long,  following  the  recent  accepted  design.  It  is 
equipped  with  spiral  scoop  feeder  and  Komata  lining,  and  makes  27  r.p.m. 
In  this  mill  there  is  but  one  passage  of  the  pulp,  there  being  no  return  of 
oversize  to  it.  It  performs  a  heavy  duty  in  reducing  the  larger  particles 
of  the  pulp,  as  the  analysis  shows. 

From  this  first  tube  mill  the  pulp  is  received  by  a  second  Dorr  classifier, 
the  slime  is  taken  out  and  joins  the  stream  from  the  first,  and  the  sand 
requiring  regrinding  is  led  into  the  second  tube  mill.  This  mill  is  the 
same  size  as  the  first  one,  but  is  equipped  with  a  smooth  cast  lining,  in 
distinction  to  the  first  which  has  a  Komata  lining.  Here  the  fine  sand  is 
ground  into  the  slime  required  for  total  pulp.  The  product  issuing  from 
the  mill  is  returned  to  the  second  classifier,  thus  forming  a  closed  circuit 
from  which  only  the  portion  light  enough  to  pass  over  the  discharge  of 
the  Dorr  classifier  issues.  The  accompanying  table  shows  the  classifica- 
tion of  the  entering  and  issuing  pulp  of  each  mill  and  also  the  classifier 
overflows. 

114 


PRACTICE  AT  TONOPAH 


115 


116 


DETAILS  OF  CYANIDE  PRACTICE 


The  Agitation  System. — From  the  regrinding  system  the  pulp  is 
taken  to  four  cone-bottom  settling  tanks,  each  24X16  ft.,  where  the  col- 
lection of  slime  is  made,  the  overflowing  solution  being  either  pumped 
back  for  further  use  or  sent  to  the  precipitation  department,  according  to 
conditions.  This  bulk  of  solution  is  used  to  maintain  the  balance 
between  precipitation  and  the  total  solution  used  in  the  mill. 

From  the  settlers  the  slime  is  drawn  off  into  the  agitation  tanks,  of 
which  there  are  four,  each  24X16  ft.,  all  equipped  with  Trent  agitators. 
At  the  Extension  mill  the  Trent  agitators  are  regarded  as  highly  efficient 
and  satisfactory  devices,  although  it  is  acknowledged  that  they  are  not 
without  their  faults.  They  do  give  some  trouble,  due  to  plugging  of  the 
nozzles,  and  it  is  also  found  that  the  bearing  upon  which  the  arms  revolve 
is  not  entirely  grit  proof  and  is  subject  to  grinding  action  of  the  pulp 

SCREEN  ANALYSES  OF  TUBE  MILL  AND  CLASSIFIER  PRODUCTS    AT 
TONOPAH  EXTENSION  MILL 


+  10 
mesh 

+20 
mesh 

+60 
mesh 

+  100 
mesh 

+  150 
mesh 

+200 
mesh 

-2  0 

mesh 

Tube  mill  No.    1  intake 
(Komata  lining). 
Tube  mill  No.   2  intake 
(smooth  lining). 
Tube  mill  No.  1  discharge  . 
Tube  mill  No.  2  discharge  . 

15.45 

1.77 
1.00 

16.90 
1.06 
0.80 

40.00 

18.08 

17.20 
2.60 

14.  OC 
35.82 

26.60 
20.60 

4.19 
14.35 

8.60 
14.00 

4.62 
13.83 

11.20 
16.80 

4.91 
15.09 

35.60 
46.00 

Classifier  No.  1  overflow. 

0  16 

1.00 

2.8 

96  81 

Classifier  No.  2  overflow. 

1.89 

5.91 

14.60 

77.60 

which  gets  into  it.  Some  experiments  have  been  made  lately  by  removing 
the  nozzles  from  the  air  and  pulp-delivery  pipes,  which  constitute  the 
arms,  and  it  has  been  found  that  while  the  speed  of  revolution  has  been 
diminished,  there  is  no  other  ill-effect  and  the  agitation  continues  as 
usual.  This  procedure  is  calculated  to  lessen  the  probability  of  stoppage 
from  plugged  nozzles.  Another  experiment  to  avoid  the  wear  occasioned 
by  grit  entering  the  bearings  and  grinding  them  out  has  been  made  by 
removing  the  bearing  altogether  from  the  tank  and  suspending  it  above 
the  tank,  connecting  it  with  the  revolving  arms  by  means  of  a  pipe.  It 
seems  probable  that  both  these  variations  may  be  productive  of  econo- 
mies, and  a  final  decision  on  the  matter  will  be  of  interest. 

The  power  required  to  move  the  Trent  agitators  is  stated  to  be  6  hp. 
for  each  agitator,  plus  the  power  required  for  furnishing  compressed  air, 
which  in  this  case  is  13  hp.  for  five  agitators,  including  the  four  treatment 
agitators  and  the  stock  tank. 

Vacuum  Filtration. — From  the  agitators  the  pulp  is  taken  to  a  stock 
tank,  30  X  20  ft.,  which  serves  to  feed  the  filter.  The  stock  tank  is  equip- 
ped with  a  Trent  agitator  to  keep  its  contents  in  suspension.  From  the 


PRACTICE  AT  TONOPAH  117 

stock  tank  the  pulp  is  taken  to  a  stationary  vacuum  filter  of  the  Butters 
type  having  100  leaves.  The  filter  is  operated  in  the  usual  way,  a  cake  of 
1J  in.  is  made  and  given  a  2-hr,  solution  wash.  The  usual  cycle  is  six 
hours  in  total.  An  additional  extraction  is  made  on  the  filter,  which  at 
times  reaches  3%.  The  slime  at  the  Extension  mill  is  easy  to  filter,  as  is 
generally  the  case  with  the  Tonopah  ores. 

Zinc  shavings  are  used  for  precipitation.  The  solution  is  clarified  by 
passing  it  through  excelsior  and  is  precipitated  in  eight  6-compartment 
zinc  boxes,  each  compartment  being  30X36X30  inches. 

The  precipitate  resulting  is  partially  dried  and  fluxed  and  melted  in 
Steele-Harvey  furnaces.  The  slag  is  allowed  to  accumulate  until  suffi- 
cient is  at  hand  for  treatment  when  it  is  crushed  in  one  of  the  batteries, 
concentrated  to  remove  the  metallic  particles  and  the  remainder  added  a 
little  at  a  time  to  the  tube  mill,  where  it  is  crushed  to  a  slime  and  cyanided 
along  with  the  regular  mill  run  of  ore.  By  treating  the  slag  in  this  man- 
ner only  a  small  portion  is  added  to  the  mill  run  every  day,  and  it  is  soon 
used  up  without  any  detrimental  effect.  It  avoids  the  expense  of  shipping 
the  material  to  smelters  and  it  is  probable  that  the  ultimate  economy  of 
the  scheme  is  as  good  or  better  than  when  it  is  shipped. 

There  is  some  controversy  over  the  relative  merits  of  zinc  shavings  and 
zinc  dust  as  a  precipitant  in  the  Tonopah  district,  and  the  operators  at  the 
Extension  mill  believe  that  the  process  using  shavings  is  as  satisfactory 
and  economical  as  when  the  dust  is  used.  They  point  to  the  fact  that  their 
consumption  of  zinc  is  as  low  as  is  usual  when  zinc  dust  is  used  and  that 
their  cost  of  cleanup  is  no  more  than  in  such  cases  and  also  that  their 
resulting  bullion  is  better  and  barren  solutions  are  easily  secured.  This 
question  is  one  which  will  stand  a  good  deal  of  argument,  and  there  seems 
to  be  something  of  advantage  for  each  system.  It  is,  however,  difficult  to 
overlook  the  great  advantage  in  simplicity  and  safety  obtained  by  the  use 
of  zinc  dust.  This  matter  will  be  referred  to  again  in  considering  the 
metallurgy  of  Tonopah  in  a  general  way. 

Extraction  Data. — The  extraction  obtained  at  the  Extension  is  consist- 
ently high  and  may  be  said  to  be  about  the  limit  obtainable  on  Tonopah 
ores.  The  material  treated  contains  silver  and  gold  in  the  proportion  of 
about  90 : 1,  the  heads  averaging  16  oz.  silver. 

The  pulp  is  given  40-hr,  agitation  in  the  tanks  with  solution  of  about 
three  pounds  of  KCN  per  ton.  The  extraction  obtained  averages  about 
94  to  95  %  of  the  total  content.  As  an  example  of  the  division  of  extrac- 
tion a  typical  month  showed  25.77%  in  milling,  67.57%  in  agitation  and 
1.74%  in  the  stock  tank  and  in  filtering.  Solutions  are  warmed  at  this 
mill,  as  is  the  general  practice  at  all  Tonopah  mills,  to  120°,  and  it  is 
claimed  that  a  clear  and  definite  additional  extraction  is  obtained  over 
working  with  normal  temperatures. 


118 


DETAILS  OF  CYANIDE  PRACTICE 

No  4-  Kennedy  Gyratory  Crusher 

Inclined  Belt  Conveyor 
Hill  Bin 


.  ,      ,                               (I                50  Stamps  105V  Ib.-l  drop,  lOOdroi 
1-24x16  Solution        J  permin.4-l2meshand2-3meshto 

Tank                Joooool        cap  screens.  Suspended  Challenge 

9"" 

H|                I-  Dorr  Duplex  Classifier 

1-  5x/8  '  Tube  Mill 

| 
\ 

y 

1-  Dorr  Duplex  Classifier 
V 

\ 

n 

1-5x18  Tube  Mil  f 

^-7~T"* 

'  />^V5- 
ES^J-v  J^/" 

Intermedi-  ^  —  .£ 

Tl  
frj'^)                4  -  <?4^/6  'Cone  bottom  seWers 

<arfe  Solution  Tank  v                   i 

i          ^          /^\ 

(    jf  )               4~24xl6  Agitation  Tank 

^      V__J/                                         Trent  Agitators 

^    T' 

ran*        j      |    ^ 

X'^^v                1-30x20'  StockTanh,  Trenf 
J                                                       Ag_itator> 

IA  ^~r^ 

;fi 

1-100  Leaf  Butters  Filter 

)Y«fer 

—  — 

1          \~^  Slime  to  Waste 
i                          ^ 

t 
i 

1 
1 

Ol-30x8  'Pr.eqnant  Solution 
Tank 

1 

1 

i 

I 

1 

r± 

i 

i 

8~6  Compartment  ZmcBoxes 

| 

§ 

! 
"o  1 

1 

g 

$Jt 
>^  —  . 

UJ_..i<^ 

-/          J              2-26x8'  Sumps 

SlagCrushed       Q^Steele -Harvey  Furnaces 
andConcent rated  j 

|     *"    ^' >  Bull  ion 

^~~7     Silver 

Residue  to 
Tube  Mill 

OreandPutp 


Solution        

FLOW  SHEET  OF  TONOPAH  EXTENSION  MILL. 


PRACTICE  AT  TONOPAH 


119 


The  consumption  of  material  is  as  follows:  Cyanide,  2.78  lb.;  lead 
acetate,  0.963  lb.;  lime,  3.398  lb.,  and  pebbles,  4.423  lb.,  all  calculated 
per  ton  of  ore  milled.  The  consumption  of  zinc  is  0.052  lb.  per  oz.  of 
bullion  recovered.  The  mill  treats  about  155  tons  of  ore  per  day  with  400 
hp.,  and  employs  14  men  regularly,  including  the  superintendent.  The 
accompanying  flow  sheet  shows  the  details  of  the  system  followed. 

An  Installation  of  Heavy  Stamps. — The  mill  of  the  MacNamara 
Mining  Co.  is  one  of  the  most  recent  of  the  Tonopah  installations,  and 
originally  contained  a  number  of  innovations,  most  of  which,  however, 
have  made  room  for  devices  conforming  to  standard  practice. 


THE  MACNAMARA  MILL. 


The  ore  is  brought  into  the  mill  over  a  conveying  and  sorting  belt,  and 
that  part  of  the  waste  which  is  easily  recognizable  is  thrown  out.  A 
Kennedy  gyratory  crusher  is  used  for  primary  crushing  and  the  ore  is  then 
dropped  into  the  battery  bins. 

There  are  10  stamps  in  the  mill,  fed  by  suspended  Challenge  feeders, 
the  stamps  weighing  1400  lb.  each.  They  are  set  in  a  special  heavy-duty 
mortar,  the  screen  being  only  three  inches  from  the  dies.  This  type  of 
mortar  is  designed  for  rapid  crushing  and,  in  fact,  at  the  MacNamara  mill 
a  duty  of  7.5  tons  per  stamp  per  day  is  attained.  With  stamps  of  this 
weight  this  could  not  be  considered  an  exceptionally  high  duty,  but  the 
crushing  capacity  is  limited  by  the  following  regrinding  installation  which 
will  not  permit  any  greater  output.  The  stamps  make  98  drops  per  min- 
ute through  8  in.,  and  the  mortars  are  equipped,  one  with  8-mesh  and  the 
other  with  12-mesh  screens.  The  different  screens  are  used  with  the  idea 


120  DETAILS  OF  CYANIDE  PRACTICE 

of  furnishing  a  varied  feed  for  the  tube  mill,  as  has  been  mentioned  in  the 
case  of  the  Tonopah  Extension  mill,  although  the  efficiency  of  this  pro- 
cedure in  the  case  of  the  MacNamara  would  seem  to  be  limited  by  the  use 
of  a  single  tube  mill. 

The  tube  mill  used  is  5  ft.  diameter  and  16  ft.  long,  which  is  carrying 
the  shortness  to  the  extreme.  As  has  been  mentioned,  the  tendency  at  the 
present  time  is  to  limit  the  length  of  the  tube  mill,  and  this  instances  the 
progression  of  that  tendency.  It  is  stated  that  with  the  16-ft.  mill  the 
results  are  the  same,  as  far  as  fineness  of  output  is  concerned,  as  if  the  22-ft. 
length  were  used  and  the  wear  on  the  lining  and  the  power  and  pebble 
consumption  are  less.  This  mill  is  equipped  with  smooth  cast  lining  and 
runs  at  26  r.p.m.  and  uses  about  60  hp.  for  its  operation.  The  cast  lining 
is  stated  to  wear  11  months,  and  as  it  is  locally  cast,  hard  iron,  the  renewal 
cost  is  not  high.  The  mill  is  equipped  with  a  scoop  feeder  of  the  spiral 
type.  The  discharge  is  taken  up  by  an  8  X  52-in.  Frenier  pump  and  re- 
turned to  the  classifier,  making  a  closed  circuit. 

The  Agitation  System. — The  slime  from  the  classifier  is  collected 
and  thickened  in  a  Dorr  thickener,  26X12  ft.,  the  overflow  solution 
being  returned  for  battery  use  and  the  thickened  slime  proceeding 
to  the  agitators.  There  are  two  agitator  tanks,  each  26 X 16  ft.,  equipped 
with  Trent  agitators'  actuated  by  4-in.  centrifugal  pumps  of  the  local  make 
already  mentioned.  Originally  the  arrangement  of  these  agitators  was 
somewhat  different,  the  bearing  was  situated  outside  and  below  the  bot- 
tom of  the  tank  and  connected  with  the  revolving  arms  by  means  of  a 
pipe  which  also  revolved.  This  pipe  passed  through  a  stuffing  box  in  the 
bottom  of  the  tank,  but  it  was  found  that  when  the  gland  was  tightened 
sufficiently  to  prevent  leakage,  the  free  revolution  was  prevented  and  the 
agitator  was  likely  to  stop  at  any  time.  Recourse  was  had  to  the  stand- 
ard Trent  device  and  subsequent  trouble  has  been  confined  to  the  points 
previously  mentioned. 

Agitation  in  these  tanks  is  continued  for  a  total  of  55  hr.,  which 
includes  the  time  required  for  filling  the  tank.  The  filling  time  is  18  to 
20  hr.,  during  which  the  agitators  are  in  use  and  the  pulp  receives  efficient 
agitation.  A  2-lb.  KCN  solution  is  used  for  treatment. 

After  the  completion  of  agitation  the  pulp  from  the  agitators  is 
received  in  a  stock  tank,  27.5X19.5  ft.,  also  equipped  with  a  Trent  agi- 
tator. From  this  tank  the  pulp  is  drawn  off  to  a  50-leaf  vacuum  filter  of 
the  Butters  type.  In  the  construction  of  the  tank  for  this  filter  it  was 
intended  to  discharge  it  from  a  single  point,  and  the  bottom  was  not 
divided  into  separate  hoppers,  but  consisted  of  a  single  V-bottom,  in 
which  was  placed  a  spiral  screw  conveyor  to  convey  the  discharged  slime 
to  the  point  of  exit.  This  arrangement  was  found  to  be  decidedly  unsat- 
isfactory, as  the  slime  stuck  and  piled  up  on  the  side  of  the  tank  and  even- 


PRACTICE  AT  TONOPAH 


121 


1-24x12' 

Batf-erySolutio 
Tank 


Overflow 


1-20x10' 
Water  Tank 

l-20xIO'Wash 
Solution        \^/ 
Tank 


•1  \ 


Co nve  yirtg  and Sor  ting  Be  I 


Kennedy  by ratory Crusher 

200  Ton  Battery  Bin 
Suspended  Challenge  Feeders 

10-1400  Ib.  Stamps,  98  drops 
&"drop.8and  12,  mesh  screens 

I- Dorr  Duplex  Classifier 


Cast  Liner 
f-8fJf  52  "prenier  Pump 

9  t 

u'ckener 


VM         2- 26  x!6rff  Trent  Agitator 
^     V  Tanks  4'Centri-fuga! 

Pumps 


Treat  Agitator 
1-SOLeafBu-fters  F//tei 


I*  I         \Slfmetoqasfe 

l~2Qx!2  PregnantSolution 


I! 
t 

1 
I 

I- 2 Ox  I 2' Lower  (  ) 

Battery  Tank  \J 


1-  Excelsior  C/ar/fier 


-6  -Compartment  Steel 
Zinc  Boxes 


1-20x12  Barren  Solution 
Sump 


Slag  Crushed 
and  Screened 


Residue 
Ore  and  Pulp 


(\       l-Steele  -  Harvey  Furnace 
I  ^ L— »•  Bui /ton 


FLOW  SHEET  OF  MACNAMARA  MILL. 


122  DETAILS  OF  CYANIDE  PRACTICE 

tually  the  screw  was  tunneling  under  the  mass  of  slime.  In  order  to 
remedy  this  defect  the  bottom  was  divided  up  into  separate  hoppers  by 
inclined  partitions,  and  is  now  conforming  to  usual  construction. 

The  filter  cycle  occupied  a  total  of  4J  hr.,  which  includes  a  2-hr, 
solution  wash  and  5-  to  10-min.  water  wash  as  final  treatment.  The  cake 
is  formed  in  from  60  to  80  min.,  and  usually  has  a  thickness  of  about  1J 
in.  In  common  with  most  Tonopah  slimes,  this  cake  is  easily  washed. 

Precipitation  on  Zinc  Shavings. — In  accordance  with  the  usual  Tonopah 
practice,  zinc  shavings  are  used  for  precipitation,  there  being  four  6-com- 
partment  boxes  preceded  by  a  special  box  containing  excelsior  for  clarify- 
ing the  solutions.  It  is  to  be  noted  that  several  plants  in  Tonopah  are 
using  this  excelsior  clarifying  system  and  it  seems  to  do  the  work  in  a 
satisfactory  manner.  It  is  undoubtedly  a  simple  and  cheap  process  and 
for  small  plants  seems  to  be  a  cheaper  method  than  installing  clarifying 
presses.  A  box  is  made  on  the  principle  of  the  zinc  precipitator  box,  the 
size  varying  with  the  requirements,  and  is  filled  with  excelsior  through 
which  the  solution  passes  in  the  same  manner  as  thourgh  a  zinc  box.  The 
excelsior  is  packed  rather  tightly  and  effectually  prevents  suspended 
slime  from  entering  into  the  zinc  box.  The  excelsior  is  easily  and  cheaply 
cleaned  when  there  is  sufficient  accumulated  slime  to  make  cleaning 
necessary. 

The  precipitate  is  partially  dried,  fluxed  and  melted  in  a  Steele-Harvey 
furnace,  the  slag  crushed  and  screened  to  separate  the  metallics  which  are 
returned  to  the  bullion  melts.  A  flow  sheet  of  the  mill  is  given,  showing  in 
detail  the  metallurgical  procedure. 

The  extraction  secured  is  90  to  92%,  in  conformity  with  that  secured 
by  most  of  the  other  tube  mills  of  the  district.  The  consumption 
of  cyanide  is  about  1J  to  If  lb.,  and  of  lime  about  three  pounds. 
The  lime  is  added  to  the  classifier  in  dry  form.  Of  lead  acetate  the 
consumption  is  0.55  lb.  per  ton  milled,  and  of  pebbles,  four  pounds 
per  ton  milled.  The  mill  crew  consists  of  seven  men,  not  including  the 
superintendent. 

The  MacNamara  is  the  smallest  mill  in  the  district  and  its  operating 
costs  are  not  to  be  fairly  compared  to  those  obtained  by  the  larger  mills. 
It  is  stated,  however,  that  the  costs  are  not  materially  more  than  those 
of  the  mills  which  most  nearly  approximate  its  capacity.  The  mill  is 
situated  in  the  town  of  Tonopah,  and  is  convenient  to  sources  of  supplies 
and  finds  it  easy  to  secure  labor  sufficient  for  its  needs.  The  details  of 
the  treatment  are  shown  in  the  accompanying  flow  sheet. 

Separate  Treatment  at  the  Desert  Mill. — Besides  the  mills  in  Tono- 
pah proper,  there  are  two  that  are,  or  have  been,  milling  Tonopah  ores 
with  success.  The  most  important  of  these  is  the  mill  of  the  Desert 
Power  &  Mill  Co.,  at  Millers,  13  miles  from  Tonopah.  This  has  been 


PRACTICE  AT  TONOPAH  123 

reducing  ores  of  the  Tonopah  Mining  Co.  for  some  years,  it  being  the 
original  cyanide  mill  of  the  Tonopah  district,  constructed  in  1906.  The 
mill  contains  100  stamps  of  1050  Ib.  each  and  regrinding  is  performed  in 
Huntington  and  Chilean  mills  of  the  Monadnock  type.  The  ore  is 
crushed  in  solution,  as  is  the  practice  in  all  mills  of  the  Tonopah  district, 
and  is  concentrated  on  Wilfley  tables  in  order  to  recover  the  sulphides 
which  are  not  so  easily  cyanided. 

The  sand  and  slime  at  this  mill  are  treated  separately  after  the  older 
method  in  use  before  treatment  as  total  slime  became  universal.     In 


MILL  AND  SAND  TAILING  PILE  OF  THE  DESERT  POWER  &  MILL  CO.,  MILLERS,  NEV. 

this  plant  the  slime  is  treated  in  tanks  having  mechanical  agitation  by 
means  of  revolving  arms  moved  by  gears  at  the  top  of  the  tanks.  The 
sand  is  subjected  to  double  treatment,  being  collected  in  33X8-ft. 
leaching  tanks  and  afterward  removed  by  means  of  Blaisdell  excavators 
and  distributed  with  distributors  of  the  same  make  into  secondary 
leaching  tanks  of  the  same  type  and  size. 

It  is  noteworthy  that  although  crushing  is  in  solution  and  concentra- 
tion is  practiced  in  the  same  solution,  the  amount  of  metal  dissolved  dur- 
ing these  operations  is  proportionally  small,  and  it  is  found  necessary  to 
take  advantage  of  every  possible  means  of  increasing  the  dissolution  of  the 
metal  after  that  point. 


124 


DETAILS  OF  CYANIDE  PRACTICE 

O)       No.  7j  Gates  Gyratory 
v     )  *  Crusher 


26   Belt  Elevator 
Trommel  I^Ho/es 


O  )          2-  No.  4  Gates  Crushers 


18  Be  It  Conveyor 
Robbins  Ore  Tripper 


1500  Ton  Storage  B 
20-Challenge  Feeder 
100  Stamps  1050 1 b. 


50  Sand  Wheel 
20"Cone  Classifiers 


3-5' Huntingdon  Mills  and 
2-6'ChileanmHs 


18  Belt  Elevator 
14  Wilfley  TablS 


10  Deli  Elevator 
hS^ Huntingdon  Hill 
30 'Sand  »hee/ 
20"Cone  Classifiers 


Butters  Distributer 
4-33x8'Cotlecting.  Tanks 
Excavtrfarand  BeltConve  yo 


(--.-..„£>)    18 -33* 8  Leach ing  Tanks 
Dump 


FLOW  SHEET  OF  DESERT  MILL. 


PRACTICE  AT  TONOPAH  125 

The  plant  in  general  is  much  like  the  South  African  plants  of  an 
early  period,  containing  tailing-wheel  elevators,  separate  treatment  with 
its  consequent  large  area  of  tanks  for  leaching  and  slime  treatment,  and 
comparatively  great  area  under  the  mill  roof.  The  mill  is  extremely  inter- 
esting as  a  comparison  with  modern  mills  of  the  same  or  greater  capacity 
now  operating  in  Tonopah  and  a  flow  sheet  of  it  is  herewith  presented. 
The  mill  treats  500  tons  per  day,  of  which  60%  is  treated  as  slime 
and  the  remainder  as  sand,  the  sand  requiring  about  15  days'  leaching  to 
extract  the  maximum  economical  percentage.  It  requires  770  hp.  and 
64  men  for  its  operation. 

Detailed  descriptions  of  the  Desert  mill  have  been  published  in  the 
technical  press,  and  it  is  not  necessary  to  review  its  methods  in  detail  at 
this  item,  as  they  are  sufficiently  well  known.  It  would,  of  course,  be 
impossible  for  a  mill  of  this  kind  to  beneficiate  ores  at  a  cost  comparable 
to  that  attained  in  modern  mills,  but  it  may  be  said  that  for  a  mill  of  its 
type,  the  Desert  plant  is  still  performing  good  work.  A  comparison  of 
the  Desert  mill  with  the  new  Belmont  plant  forms  a  good  object  lesson  a? 
to  what  great  changes  have  taken  place  in  the  cyanidation  of  silver  ores 
within  six  years. 

At  Millers  there  is  also  the  old  mill  of  the  Belmont  company,  having 
60  stamps  and  built  after  the  separate  treatment  plan,  much  like  the 
Desert  mill.  This  mill  is  now  treating  a  small  quantity  of  custom  ore 
and  retreating  some  of  its  sand  tailing.  A  description  of  it  would  be 
unnecessary  at  this  time. 

DISCUSSION 

Cost  of  Cyanidation  at  Tonopah 

In  the  synopsis  of  Herbert  A.  Megraw's  article  on  "  Silver  Cyanidation 
at  Tonopah — II,"  in  the  JOURNAL  of  Mar.  1,  with  reference  to  the  Desert 
mill  at  Millers,  Nev.,  the  statement  is  made  that,  "  Costs  are  higher  than 
those  obtained  with  modern  all-slime  plants,"  and  in  the  text  of  the  arti- 
cle, one  reads:  "It  would,  of  course,  be  impossible  for  a  mill  of  this  kind 
to  beneficiate  ores  at  a  cost  comparable  to  that  attained  in  modern  mills." 

It  is  possible  that  what  I  believe  to  be  an  erroneous  impression  may  be 
gained  from  the  above  statements.  If  Mr.  Megraw  refers  to  the  modern 
mills  of  Mexico  or  modern  mills  of  the  United  States,  treating  straight 
gold  ore,  the  statements  are  correct,  but  if  he  would  only  refer  to  the  mills, 
both  modern  and  antiquated,  of  the  Tonopah  district,  which  would 
be  a  fair  basis  for  a  comparison  of  costs,  I  believe  an  investigation 
would  show  that  the  operating  costs  of  the  Desert  mill,  granted  to  be 
antiquated,  have  been,  and  are  still,  lower  than  those  of  any  other 
mill  in  the  district. 


126  DETAILS  OF  CYANIDE  PRACTICE 

For  the  eleven  months  of  the  present  fiscal  year  the  direct  costs  at 
the  Desert  mill  are,  $2.489  per  ton,  indirect  $0.146,  total  $2.635  per  ton. 
Direct  costs  include  all  labor,  power  and  supplies  used  in  operating  the 
mill  together  with  all  costs  of  repairs,  renewals,  and  upkeep  of  plant  and 
equipment.  Taxes,  insurance,  legal  and  home  office  expense  are  included 
in  the  indirect  costs. 

A.  R.  PARSONS. 

Tonopah,  Nev.,  Mar.  8,  1913. 

It  is  possible  that  Mr.  Parsons  has  misinterpreted  the  statements  to 
which  he  refers  in  my  article  dealing  with  cyanide  practice  at  Tonopah. 
What  I  meant  was  that  a  plant  operating  on  the  old  separate-treatment 
system,  without  the  most  modern  machinery  and  appliances,  cannot 
possibly  reach  the  low  level  of  cost  attainable  in  all-slime  plants  where 
every  improvement  is  at  hand.  Of  course,  to  make  any  comparisons  of 
value,  the  scale  of  operations  must  be  approximately  the  same,  and  for 
this  reason  the  Desert  mill,  treating  500  tons,  cannot  be  compared  to  the 
other  mills  in  Tonopah  which  handle  150  tons  or  less  per  day.  An  instruc- 
tive contrast  might  be  made  between  the  Desert  mill  and  the  new  Belmont 
mill,  which  treats  approximately  the  same  tonnage  of  the  same  kind  of 
ore.  Undoubtedly  the  Belmont,  a  modern  and  efficiently  operated 
installation,  is  treating  its  ore  at  less  cost  than  the  Desert  mill,  or  will  be 
doing  so  when  normal  conditions  can  be  maintained.  My  statement  was 
not  limited  to  the  Tonopah  district  alone. 

I  am  aware  of  the  cost  of  milling  at  the  Desert  mill  and  consider  it 
exceptionally  low  for  a  mill  of  that  kind.  I  think  Mr.  Parsons  will  agree 
with  me,  however,  in  believing  that  a  mill  equipped  with  every  modern 
improvement  can  treat  the  same  ore  as  efficiently  and  at  less  cost. 

H.  A.  MEGRAW. 

New  York,  Mar.  18,  1913. 

On  reading  Mr.  Megraw's  article  on  "  Silver  Cyanidation  at  Tonopah 
— II"  in  the  JOURNAL  of  Mar.  1,  a  reference  to  the  slime  discharge  of  the 
MacNamara  mill  will  be  noted,  which  seems  to  condemn  that  type  of 
installation  generally  as  a  method  of  removal  of  tailings. 

Quoting  briefly,  he  says:  "The  mill  of  the  MacNamara 

originally  contained  a  number  of  innovations,  most  of  which  have  been 

made  for  devices  conforming  to  standard  practice The 

filter  bottom  was  not  divided  into  separate  hoppers,  but  consisted  of  a 
single  V-bottom,  in  which  was  placed  a  spiral  screw  conveyor  to  convey 
the  discharged  slime  to  the  point  of  exit.  The  arrangement  was  found  to 
be  decidedly  unsatisfactory,  as  the  slime  stuck  and  piled  up  on  the  side  of 
the  tank  and  eventually  the  screw  was  tunneling  under  the  mass  of  slime. 


PRACTICE  AT  TONOPAH  127 

In  order  to  remedy  this  defect  the  bottom  was  divided  up  into  separate 
hoppers  by  inclined  partitions,  and  is  now  conforming  to  usual  con- 
struction." 

Though  without  first-hand  knowledge  of  the  conditions  prevailing 
at  the  MacNamara,  I  wish  to  state  that  this  failure  of  a  screw  conveyor 
readily  to  clean  a  filter  hopper  is  not  to  be  considered  a  typical  perfor- 
mance, nor  should  it  be  a  sufficient  reason  to  revert  to  the  common 
practice. 

As  evidence,  I  would  submit  the  results  obtained  at  El  Tigre,  Sonora, 
where  a  screw  conveyor  is  transporting  heavy  filter  slime-cake  that  is 
dropped  into  a  hopper  that  contains  no  water  whatever.  The  equip- 
ment, as  first  set  up,  was  a  screw  of  ribbon  type,  revolving  in  the  bottom 
of  a  horizontally  set  discharge  hopper,  the  sheet-iron  walls  of  which  are 
inclined  at  an  angle  of  45°.  The  residues  discharged  are  filter  cakes  from 
Kelly  presses,  which  range  from  25  to  30%  moisture,  and  more  often 
approach  the  former  figure  than  the  latter.  Inasmuch  as  the  cakes  are 
stiff  and  sticky,  the  leaves  are  washed  with  water  from  a  high-pressure 
hose,  after  having  dropped  the  cake  by  internal  water  pressure,  but  the 
total  discharge  from  the  hopper  has  not  exceeded  a  moisture  ratio  of 
0.5  of  water  to  1.0  of  solids,  more  frequently  being  0.  4  :1,  and  there 
is  no  accumulation  of  water  in  the  hopper  at  any  time,  to  serve  as  a 
flushing  medium. 

When  first  put  in  use,  the  conveyor  behaved  as  described  by  Mr. 
Megraw,  and  " tunneled"  through  the  slime-cake,  making  the  prompt 
discharge  of  residues  impossible.  After  some  time  elapsed,  the  following 
alterations  were  made,  which  changed  the  screw  from  a  source  of  annoy- 
ance and  lost  time,  to  a  smoothly  working  mechanism.  First,  the  bottom 
plates  were  taken  out,  and  a  trough,  22  in.  wide,  was  bult  into  the  bottom, 
of  similar  gage  iron.  The  hopper  bottom  was  originally  level,  but  this 
trough  has  an  inclination  of  12%  on  a  50-ft.  length. 

The  ribbon  shaft  was  lowered  as  well,  but  not  as  much,  and  is  now  a 
foot  above  the  bottom  of  trough  at  discharge  end,  while  at  the  other  ex- 
treme it  is  only  'about  5  or  6  in.  above  it. 

The  result  of  the  change  is  that  all  the  slime  slides  into  the  trough  and 
is  at  once  cut  up  by  the  worm,  and  pushed  forward  before  it  has  time  to 
lag  and  hold  back  the  succeeding  fragments  of  cake  that  fall.  The  small 
amount  of  hose  water  that  is  used  acts  as  a  lubricant  and  the  slime  cakes 
shoot  out  of  the  discharge  end  of  the  hopper  as  though  on  a  greased 
skid  way. 

No  fall  or  head  room  was  sacrificed,  as  the  hopper  is  at  the  head  of  the 
tailings  ditch.  In  passing,  it  might  be  said  that  without  the  aid  of  the 
screw  conveyor,  it  is  impossible  to  discharge  the  hopper  save  by  long-con- 
tinued sluicing  with  much  water.  The  conveyor  is  driven  by  a  small 


128  DETAILS  OF  CYANIDE  PRACTICE 

motor  that  takes  about  3  hp.  under  the  new  arrangement,  and  as  it  is  in 
operation  about  4  hr.  in  all  per  day,  the  power  requirements  are  extremely 
small. 

Cake  discharges  are  now  made  in  1.5  to  18  min.,  counting  from  the 
time  that  the  presses  are  opened  until  the  time  that  they  are  beginning  to 
fill  again  with  slime.  In  this  time  five  presses  of  the  "Parral"  size  are 
cleaned.  Occasional  discharges  have  been  made  in  10  or  12  minutes. 

The  above  statement  shows  that  screw  conveyors  will  remove  filter 
slime  under  the  proper  conditions,  even  though  much  denser  than  the 
average  vacuum  filter_cake,  and  at  no  noticeable  expense  for  power  or 
repairs. 

Conditions  at  El  Tigre  preclude  the  use  of  much  water  or  water  in 
large  quantities  at  recurring  intervals.  At  other  plants  they  might  be 
such  as  to  offer  no  field  for  the  screw  conveyor  for  a  variety  of  reasons, 
but  it  is  certainly  possible  to  make  it  work  well  as  a  means  of  transport  for 
slime  cake  in  a  filter  plant.  Furthermore,  it  does  not  leave  masses  of 
heavy  slime  in  the  corner  of  hopper,  as  is  frequently  the  case  when  using 
rectangular  boxes  and  dropping  the  cake  in  water  or  barren  solution. 

DONALD  F.  IRVIN.  . 

Yzabal,  Sonora,  Mar.  15,  1913. 


CHAPTER  XII 
PRACTICE  AT  TON OPAH— Concluded 

The  ores  of  the  Tonopah  district  have  not  as  yet  been  subjected  to 
thorough  tests  in  order  to  determine  the  advisability  of  sorting  the  ore 
before  milling  it.  Some  of  the  plants  have  adopted  sorting  in  a  desultory 
way  and  two  have  taken  some  pains  to  accomplish  a  more  or  less  thorough 
sorting.  These  latter  are  the  West  End  Consolidated  and  the  new  Bel- 
mont.  The  first  of  these  has  installed  a  mechanical  washing  machine 
which  screens  and  washes  the  ore  in  hot  water  and  then  delivers  it  upon  a 
wide  sorting  belt  running  at  slow  speed.  This  belt  is  so  arranged  that 
the  sorters  work  on  one  side  of  it,  throwing  the  waste  not  off  the  belt,  but 
to  one  side  of  it.  At  the  end  of  this  belt  there  is  a  splitter  which  bivides 
the  stream  delivery,  the  milling  ore  going  to  its  proper  bin  while  the 
waste  is  delivered  into  a  waste  bin  from  which  it  can  be  readily  discarded. 

There  are  several  advantages  about  this  sorting  system.  The  ore  to  be 
sorted  is  passed  over  a  trommel  with  small:  holes  and  is  given  a  thorough 
rasping  which  dislodges  much  of  the  sticking  clay  and  mud  and  also  gets 
rid  of  the  fine  portion  of  the  ore  which  does  not  require  sorting  but  which 
goes  directly  into  the  mill.  The  coarser  ore  thus  is  passed  through  into 
the  washer  proper,  which  is  a  large  steel  cylinder  set  on  an  incline  with  its 
lower,  or  receiving  end,  set  into  a  tank  of  water  which  is  heatedly  steam. 
The  inside  of  the  cylinder  contains  a  spiral,  or  reverse  screw  arrangement, 
which  obliges  the  ore  to  ascend  through  the  cylinder  until  it  is  finally  dis- 
charged at  the  higher,  or  discharge  end.  Near  the  discharge  end  there  is 
a  section  of  the  cylinder  perforated  with  small  holes  through  which  the 
washing  water  returns  to  the  tank  below.  The  coarse  ore  delivered  to  the 
sorting  belt  is  in  perfect  condition  for  sorting,  as  the  operator  can  see  at  a 
glance  its  character  and  can  eliminate  all  absolute  waste  without  hesita- 
tion. The  mud  and  clay  washed  from  the  ore  are  collected  in  the  wash- 
ing tank  and  can  be  recovered  at  convenient  intervals.  It  is  usually  of 
high  grade  and  is  sent  to  the  mill  along  with  the  regular  milling  ore. 

The  practical  utility  of  this  system  has  been  thoroughly  tested  and 
calculated  at  the  West  End  plant  and  the  conclusion  is  that  there  is  a 
clear  and  definite  saving  by  its  use.  A  large  portion  of  rock  that  will 
not  pay  its  way  'through  the  mill  is  discarded,  saving  the  milling  cost  of 
putting  it  through  the  process  and  enriching  the  portion  of  payable  ore. 
In  other  words,  the  same  amount  of  silver  is  concentrated  into  a  smaller 

129 


130 


DETAILS  OF  CYANIDE  PRACTICE 


bulk  for  milling,  the  economies  being  evident.     The  new  Belmont  mill 
has  installed  a  similar  machine  for  washing  the  ore. 

Sorting  in  some  form  is  practiced  by  several  of  the  operating  companies 
at  Tonopah,  but  only  at  the  above  mentioned  plants  is  it  carried  out  in 
thorough  form.  It  has  been  stated  that  a  great  many  of  the  Tonopah 
dumps  would  pay  well  for  sorting  them  over  if  it  were  possible  to  lift 
the  material  economically  and  get  rid  of  the  waste.  If  this  statement  is 
true,  it  seems  likely  that  sorting  while  the  ore  was  originally  in  motion 
might  have  repaid  the  trouble  necessary  to  do  it.  It  is  a  question  which 
requires  some  study  and  it  is  impossible  to  make  any  statement  which 
would  certainly  cover  all  cases. 


NEW  MILL  AT  TONOPAH,  NEV.,  AND  OLD  MILL  AT  MILLERS,  OF  THE  TONOPAH- 
BELMONT  DEVELOPMENT  CO. 

Crushing  in  Solution. — All  the  operating  mills  at  Tonopah  use  solu- 
tion throughout  the  mill  operations.  At  the  same  time  there  are  certain 
of  the  operators  who  believe  that  better  metallurgical  results  could  be 
obtained  if  it  were  possible  to  do  the  crushing  and  grinding  in  water 
without  incurring  a  loss  of  cyanide  and  dissolved  metal.  It  is  evident 
that  in  treating  silver  ores  where  solution  of  comparatively  high  cyanide 
content  must  be  used,  crushing  in  water  is  not  usually  practicable.  The 
amount  of  water  taken  in  unavoidably  from  the  grinding  system  to  the 


PRACTICE  AT  TONOPAH  131 

cyanide  plant  would  have  to  be  discharged  as  moisture  with  the  residues, 
and  at  that  time  would  contain  both  cyanide  and  metal  in  solution,  a 
combination  that  could  not  be  thrown  away.  The  only  system  by  which 
it  might  be  managed  would  be  to  filter  the  pulp  from  the  grinding  system, 
whereby  it  could  be  introduced  into  the  cyanide  plant  with  approximately 
the  same  quantity  of  water  as  the  residues  would  contain  in  moisture. 
Whether  the  advantages  obtainable  would  justify  the  additional  expense 
is  a  question  which  would  have  to  be  solved  in  each  particular  case.  The 
procedure  is  followed  at  the  mill  of  the  Smuggler-Union  Mining  Co.,  at 
Telluride,  Colo.,  as  has  already  been  mentioned  in  these  papers. 

Crushing  in  Water  at  Goldfield. — The  advantage  of  crushing  in  water 
is  believed  to  be  a  saving  of  cyanide  by  removing  cyanicides  from  the  ores, 
resulting  also  in  an  improved  extraction.  This  matter  has  received 
much  attention  from^the  operators  at  the  Goldfield  Consolidated  mill 
and  experiments  covering  several  months'  work  have  convinced  them  that 
water  crushing  has  decided  advantages  over  crushing  in  solution.  Natu- 
rally enough,  the  conditions  at  the  Goldfield  mill  are  decidedly  different 
from  those  obtaining  at  Tonopah  and  the  results  of  experiments  cannot  be 
transferred  bodily.  The  former  is  treating  a  gold-bearing  ore  and  using 
solutions  much  weaker  than  those  necessary  for  the  ores  of  Tonopah. 

At  the  Goldfield  Consolidated,  it  has  been  satisfactorily  proved  that 
when  crushing  in  water  is  followed  the  consumption  of  cyanide  is  less  and 
the  extraction  of  metal  better  than  when  crushing  in  solution.  It  is 
shown,  also,  that  when  crushing  in  solution  is  practiced,  there  is  likely  to 
appear  in  the  mill  a  light  gelatinous  product  which  refuses  to  settle  and 
causes  a  great  deal  of  trouble  in  mill  operating.  The  substance  appears 
much  like  an  alumina  product,  which  it  may  be,  but  that  has  not  been 
proved.  On  the  contrary,  when  water  is  used  in  the  crushing  system,  this 
product  does  not  appear. 

It  is  extremely  difficult  to  account  for  this  difference  and  many  theories 
have  been  invented  in  the  attempt  to  do  so.  The  theory  has  been 
advanced  that  no  matter  how  alkaline  a  crushing  solution  may  be,  and  all 
solutions  containing  cyanide  must  be  kept  alkaline,  there  will  be  a  certain 
length  of  time,  however  short,  when  the  acid  liberated  from  the  ore  will 
overbalance  any  alkalinity  immediately  available  at  any  one  point,  and 
that  during  this  short  time  a  chemical  change  might  take  place  which 
would  render  soluble  some  elements  which  would  not  be  put  into  solution 
with  water  and  which  would  be  removed  by  washing  before  reaching  the 
cyanide  department,  were  water  used  in  crushing.  This  momentary 
acid  condition  might  also  introduce  certain  combinations  prejudicial  to 
high  extractions.  Of  course,  there  has  been  no  research  which  would 
prove  that  this  condition  does  exist  and  is  responsible  for  the  results 
shown,  but  the  results  are  plain*  Commercial  operators  rarely  have 


132  DETAILS  OF  CYANIDE  PRACTICE 

the  opportunity  to  solve  questions  of  this  kind,  but  it  is  just  such  prob- 
lems which  most  need  solving  in  the  cyanide  field. 

Whether  or  not  similar  conditions  to  those  found  at  Goldfield  would 
obtain  at  Tonopah  is  not  certain,  but  with  this  example  in  mind  there  are 
many  who  would  like  to  have  the  opportunity  of  trying  it. 

The  use  of  gravity  stamps  for  crushing  is  universal  at  Tonopah  and  it 
seems  that  nothing  else  has  been  considered  in  the  design  of  the  plants. 
This  seems  unfortunate,  for  the  ore  does  not  appear  to  be  too  hard  to  give 
good  results  with  rolls  or  Chilean  mills,  either  of  which  might  be  applied 
with  economy.  It  is  true,  however,  that  stamps  have  been  considered  the 
conservative  crushing  machine  for  a  long  time  and  the  design  and  con- 
struction of  plants  at  Tonopah  have  been  eminently  conservative.  To 
install  a  machine  which  will  surely  do  the  work,  even  though  at  a  cost 
somewhat  higher  than  that  of  one  which  might  be  satisfactory,  is  a  sound 
basis  upon  which  to  reason  and  designers  can  hardly  be  censured  for 
following  that  system.  At  the  same  time  the  results  obtained  by  the 
other  machines,  rolls  and  Chilean  mills,  both  as  to  efficiency  and  cost,  are 
no  less  definite  and  clearly  proved,  the  obstacle  to  their  use  being  simply 
that  the  results  are  not  so  well  known.  One  cannot  say  with  certainty 
that  either  of  the  machines  above  mentioned  would  satisfactorily  take 
the  place  of  stamps  at  Tonopah,  but  it  is  not  too  much  to  say  that  I 
believe  either  of  them  would  introduce  economies. 

Lining  for  Tube  Mills. — The  matter  of  tube  mills  at  Tonopah  intro- 
duces some  interesting  variations  which  are  worthy  of  attention.  First 
attention  is  called  by  the  shortness  of  the  mills  generally  used,  especially 
in  the  later  installations.  This  progression  has  apparently  been  along 
conservative  lines  and  there  is  reason  to  believe  that  the  theory  is  sound. 
Certainly  it  has  resulted  in  a  reduction  of  power  for  the  same  production. 

The  mills  recently  installed  are  generally  5  ft.  diameter  and  18  ft. 
long,  in  one  case  the  length  has  been  reduced  to  16  ft.  with  satisfactory 
results.  It  will  be  interesting  to  watch  this  development  and  see  what 
it  brings  forth.  It  is  not  impossible  that  the  tube  mills  of  the  future  may 
be  more  like  drums  of  large  diameter  and  short  length  than  the  long, 
narrow  tubes  first  used. 

The  lining  of  the  tube  mills  has  given  rise  to  much  argument.  The 
contrast  here  to  Mexican  practice  is  noteworthy.  In  all  the  Tonopah 
installations  I  have  not  seen  a  single  instance  of  the  use  of  the  El  Oro 
ribbed  lining,  but  everywhere  there  is  the  use  of  the  smooth  cast  lining 
and  the  silex  blocks,  both  of  which  had  been  tried  and  discarded  in 
Mexico  before  the  El  Oro  lining  came  into  use.  The  smooth  cast  lining 
would  seem,  theoretically,  to  be  the  most  inefficient  of  all  possible  linings, 
as  it  would  appear  that  the  pebbles  would  be  more  than  likely  to  slide 
over  it  and  reduce  the  ore  by  rubbing  it  between  the  lining  and  the 


PRACTICE  AT  TONOPAH  133 

pebbles.  The  matter  of  the  dropping  or  tumbling  of  pebbles  would 
apparently  be  reduced  to  a  minimum,  and  while  it  is  clear  that  the 
power  requirements  would  be  reduced  to  the  lowest  possible  point, 
apparently  the  efficiency  would  go  down  with  it.  That  this  is  not  the 
case  is  what  several  of  the  operators  declare  positively.  They  claim  to 
get  better  results,  both  as  to  character  of  product,  its  quantity  and  cost 
of  production.  And  it  is  also  stated  positively  that  the  pebbles  do  not 
wear  flat  in  the  smooth-lined  mills.  It  is  a  fact  that  if  one  is  careful  he 
can  avoid  seeing  flat  pebbles  on  the  scrap  heaps  of  these  mills. 

The  silex  lining  has  its  advocates  also,  the  installation  at  the  new 
Belmont  mill  containing  silex  lining  except  in  experimental  cases.  Silex 
has  proved  to  be  a  good  lining  in  many  cases,  and  except  for  the  item  of 
cost  has  done  good  work  in  Mexico.  The  objections  there  to  its  use  are 
the  cost  and  the  time  necessary  to  reline  the  mill  when  it  wears  out. 
This  usually  puts  a  tube  out  of  commission  for  from  five  to  seven  days 
and  where  there  is  no  reserve  capacity  it  seriously  interferes  with  the  mill 
operation. 

The  Komata  lining  has  had  trials  at  Tonopah  and  in  the  case  noted 
in  the  Tonopah  Extension  mill,  is  doing  satisfactory  work.  Komata 
linings  have  also  been  installed  at  the  Goldfield  Consolidated  with 
satisfactory  results  and  are  now  used  altogether.  It  is  claimed  that 
these  linings  increase  the  power  consumption,  which  is  undoubtedly 
true,  but  that  the  ultimate  efficiency  is  increased.  This  is  even  more 
definitely  shown  when  the  revolutions  of  a  mill  into  which  they  are 
placed  are  reduced  considerably.  For  instance,  a  5Xl8-ft.  tube  mill 
using  smooth  or  silex  lining  and  revolving  at  26  or  27  r.p.m.  can  be  re- 
duced, when  the  Komata  lining  is  used,  to  about  22  or  23  r.p.m.,  resulting 
in  a  lower  power  consumption  and  no  great  reduction  of  efficiency. 

The  height  at  which  the  load  of  pebbles  is  carried  also  varies  a  good 
deal.  The  variation  is  from  somewhat  below  the  center  line  of  the 
mill  to  6  or  8  in.  above  it,  calculating  with  the  mill  not  running,  of 
course.  In  cases  where  the  load  is  carried  above  the  center  line  of  the 
mill  a  grating  has  to  be  used  in  the  discharge  in  order  to  prevent  the 
issuing  of  pebbles.  At  the  West  End  mill  a  difference  of  pebble  load 
made  an  enormous  difference  in  the  efficiency  of  the  tubes  so  that  the 
whole  mill  was  affected.  This  question  of  lining  will  probably  be 
solved  in  the  near  future  when  there  will  be  some  agreement  among  the 
operators  as  to  which  type  best  suits  their  needs.  At  present  there 
seems  to  be  a  great  difference  of  opinion. 

Types  of  Agitators. — Opinions  in  regard  to  agitators  seem  to  differ 
as  widely  as  they  do  in  tube-mill  linings.  There  are  examples  of  the 
Pachuca,  Hendryx,  Trent  and  other  systems  in  use.  The  original  mills 
of  the  district,  the  Desert  and  Belmont  mills  at  Millers,  used  mechanical 


134  DETAILS  OF  CYANIDE  PRACTICE 

agitators  for  their  slimes,  the  Hendryx  type  is  in  use  at  the  Montana- 
Tonopah  mill  and  several  installations  are  using  the  Trent  system,  while 
the  new  Belmont  mill  at  Tonopah  is  using  the  Pachuca  system. 

Particularly  the  Trent  agitator  seems  to  be  a  bone  of  contention. 
Some  users  say  it  is  perfect  and  others  delcare  it  has  no  merit  in  it,  that 
it  is  a  nuisance  and  not  satisfactory  in  any  way.  The  truth  lies  somewhere 
between  these  two  extremes,  no  doubt,  but  at  what  exact  point  is  im- 
possible to  state. 

The  Hendryx  agitator  has  been  previously  mentioned  in  these 
columns,  but  it  is  a  machine  which  has  never  become  popular  on  account 
of  its  high  power  consumption.  There  is  no  doubt  that  it  is  a  good 
agitator,  but  it  will  probably  never  become  a  serious  factor  in  the  agita- 
tion of  slime  for  the  reason  of  its  high  operating  cost. 

Pachuca  tanks  have  been  the  subject  of  investigation  for  some  time 
and  many  operators  claim  that  they  offer  no  advantages  in  economy  nor 
in  additional  extraction  obtained. 

Referring  back  to  the  new  Nipissing  mill  at  Cobalt,  in  which  the 
slime  agitators  are  the  old-style  mechanically  moved  arms,  the  designers 
stated  that  they  had  been  able  to  find  no  device  which  would  assure 
them  either  better  results  or  lower  costs.  In  such  event  the  Pachuca 
tank  would  be  entirely  out  of  the  reckoning,  for  if  it  presents  no  such 
advantages,  its  high  cost  of  installation  and  the  additional  cost  of  pump- 
ing in  a  mill,  occasioned  by  the  height  of  the  tanks,  would  certainly 
preclude  its  installation.  I  am  of  the  opinion  that  the  Pachuca  tank  is 
likely  to  be  replaced  by  some  other  more  satisfactory  design. 

Concentration  Practice. — It  will  be  noticed  that  some  of  the  Tonopah 
mills  practise  concentration  while  others  do  not.  The  two  at  Millers, 
the  Montana-Tonopah,  the  West  End  and  the  New  Belmont  practise 
concentration,  while  the  Tonopah  Extension  and  the  MacNamara  mills  do 
not.  In  those  mills  which  do  concentrate,  the  object  has  been  to  make 
as  little  concentrate  as  possible  consistent  with  removing  the  objection- 
able elements  from  the  ore.  With  low-  or  medium-grade  ores  con- 
centration is  not  necessary  but  with  the  rise  in  silver  content,  attendant 
upon  the  increase  in  the  proportion  of  sulphides,  concentration  must 
be  resorted  to.  Many  of  the  mills  using  the  process  have  reduced  the 
number  of  tables,  the  point  being  that  it  is  more  expensive  to  market 
concentrate  than  bullion;  therefore  as  large  ^a  proportion  as  possible 
should  be  taken  out  in  bullion  form.  This  is  sound  reasoning,  par- 
ticularly where  the  concentrate  has  to  be  shipped  to  smelters  where 
losses  may  be  multiplied  by  many  means. 

There  is  no  instance  of  local  treatment  of  concentrate  at  Tonopah. 
In  this  connection  it  is  interesting  to  review  the  situation  at  Goldfield, 
where  the  Consolidated  treats  all  of  its  concentrate  by  an  elaborate  proc- 


PRACTICE  AT  TONOPAH  135 

ess.  Experiments  on  concentrate  treatment  showed  that  by  treat- 
ing the  concentrate  raw  a  certain  percentage  could  be  saved;  that  by 
first  roasting  the  concentrate  and  then  cyaniding  it,  a  better  extraction 
would  result,  but  that  by  treating  it  raw,  then  roasting  it  and  retreating 
by  cyanide,  an  extraction  about  equal  to  the  sum  of  the  results  of  the 
other  two  methods  could  be  obtained.  That  system  has  been  installed 
and  is  followed  with  success.  After  the  cyanidation  of  the  roasted 
concentrate,  the  residue  is  delivered  into  the  tank  which  feeds  the  filter 
and  is  discharged  with  the  filtered  tailing.  By  this  means  the  con- 
centrate tailing  is  reduced  to  a  value  nearly  equal  to  that  of  the  regular 
mill  tailing.  The  process  is  interesting  and  unusual. 

Use  of  Heated  Solutions. — Operators  universally  agree  that  the 
extraction  of  silver  from  the  Tonopah  ores  is  increased  by  heating  the 
solutions.  The  point  to  which  this  is  carried  is  generally  about  120°. 
There  seems  to  be  no  difference  of  opinion  on  this  point  and  all  the  mills 
are  doing  this  with  good  results.  The  question  naturally  occurs  as  to 
whether  the  Mexican  operators  have  been  overlooking  anything  in  the 
matter  of  heating  solutions.  I  have  seen  the  scheme  tried  several  times 
and  made  experiments  on  it,  but  without  any  beneficial  result.  That  this 
is  not  unusual  will  be  shown  in  a  later  paper,  when  I  will  call  attention  to 
another  silver  mill  which  has  made  repeated  experiments  and  found  no 
increase  of  extraction  through  heating  the  solutions.  Still,  this  is  a 
point  which  ought  to  be  worked  out  for  each  particular  ore,  no  general 
rule  governing  it. 

Heating  solutions  seems  to  have  no  effect  whatever  on  the  extraction 
of  gold.  This  means,  of  course,  solutions  of  normal  temperature,  as 
extraction  of  both  metals  falls  off  in  abnormally  cold  solutions. 

The  use  of  lime  and  lead  acetate  at  Tonopah  does  not  vary  widely 
from  accepted  practice  at  most  other  camps.  The  addition  of  these 
materials  varies,  and  the  practice  is  governed  principally  by  convenience. 
The  filters  in  use  at  Tonopah  are  all  of  the  stationary  vacuum  type. 
The  present  condition  of  the  filter  problem  makes  it  advisable  for  dis- 
cussion to  be  postponed  until  there  shall  be  more  liberty  to  publish 
statements. 

Calculation  of  Extraction. — At  Tonopah,  as  in  most  of  the  mills  at 
the  present  time,  the  calculation  of  extraction  is  performed  by  using  the 
content  of  bullion  produced  plus  the  content  of  tailing  discharged  as  the 
value  of  head  samples.  Some  of  the  older  operators  look  with  envy  on 
the  mill  men  of  the  present  day  and  think  of  the  monthly  recurrence  of 
attacks  of  heart  disease  that  came  with  the  attempt  to  reconcile  pro- 
duction and  tailing  content  with  head  samples  taken  at  or  before  the 
batteries.  Extractions  of  120%  were  not  unusual  in  those  days,  nor  were 
drops  to  60  and  70%  unexpected,  but  the  mill  man  was  continuously  in 


136  DETAILS  OF  CYANIDE  PRACTICE 

hot  water  about  it,  particularly  if  he  differed  with  the  general  manager 
about  the  accuracy  of  the  head  samples.  This  later  method  is  much 
simpler  and  tends  to  avoid  worries.  At  the  same  time  it  is  hard  to  agree 
with  a  method  of  solving  a  problem  by  means  of  which  any  answer  is 
the  right  one.  I  am  anxious  to  have  this  matter  discussed  and  hope  that 
there  will  be  someone  kind  enough  to  take  it  up  and  extract  opinions 
about  it. 

DISCUSSION 
Silver  Cyanidation  at  Tonopah — III 

In  Mr.  Megraw's  article,  "  Silver  Cyanidation  at  Tonopah — III," 
in  the  JOURNAL  of  Mar.  8,  1913,  it  is  suggested  that  crushing  in  water 
might  be  accomplished  without  an  additional  or  prohibitive  cyanide  loss 
if  the  pulp  were  filtered  after  grinding,  introducing  it  into  the  cyanide 
plant  at  practically  the  same  moisture  contained  by  the  residue  on 
discharging.  As  the  amount  of  water  taken  into  the  mill  without 
building  up  the  quantity  of  solution  is  governed  entirely  by  the  amount 
of  moisture  contained  in  the  residue  at  the  time  of  discharging,  and  can 
only  be  equal  to  this  amount,  the  water  added  ahead  of  cyanidation  would 
preclude  the  use  of  water  for  final  wash.  It  would  allow  the  discharge  to 
go  out  containing  33%  of  standard  cyanide  solution,  which  is  in  our 
case  three  pounds  per  ton  of  ore.  When  the  cost  of  dewatering  with 
filters  is  considered  together  with  the  mechanical  loss  of  cyanide,  and 
the  trouble  and  expense  of  again  thoroughly  mixing  a  pulp  dewatered 
to  such  a  point  of  dryness,  the  additional  extraction  above  94.44%,  the 
average  obtained  by  the  Belmont  Milling  Co.  for  the  last  eight  months, 
it  would  hardly  seem  to  be  an  attractive  undertaking,  even  when  con- 
sidering the  cyanide  which  might  be  saved  by  crushing  in  water. 

As  to  the  mechanical-stirrer  type  of  agitator  being  efficient  on  silver 
sulphide  ore,  I  will  state  that  the  type  of  agitator  called  the  " Butters" 
was  originally  installed  at  our  Millers  plant  and  only  a  fair  extraction 
could  be  obtained  from  it  until  a  6-in.  air  lift  was  added,  taking  pulp 
from  bottom  of  the  vat  and  discharging  it  over  the  top,  in  addition  to  the 
mechanical  stirring. 

At  the  Tonopah  mill  where  solution  during  agitation  is  kept  at  100°, 
the  air  for  agitation  is  furnished  by  a  steam-driven  compressor  and  the 
exhaust  steam  is  used  for  heating  the  solutions.  As  there  is  only  7% 
loss  between  live  steam  at  125-lb.  pressure  and  exhaust  at  five  pounds,  in 
heating  efficiency,  we  can  figure  only  7%  of  cost  of  producing  steam 
against  agitating  medium  or  method,  so  that  if  air  were  not  beneficial 
to  extraction,  and  I  am  of  the  opinion  that  it  is,  another  method  or 
medium  could  hardly  be  had  at  the  same  or  lower  cost  per  ton. 


PRACTICE  AT  TONOPAH  137 

At  the  Millers  plant  where  live  steam  is  used  for  heating  solution,  the 
cost  averages  about  18c.  per  ton,  while  at  the  Tonopah  plant,  with,  of 
course,  a  better  equipment,  heating  and  air  for  agitation  together  amount 
only  to  12c.  per  ton  of  ore  treated. 

The  benefits  to  be  derived  from  the  heating  of  solutions  used  with 
Tonopah  ores,  are  shown  in  the  following  tailing  assays  from  tests  made 
identically,  except  in  temperature.  They  typify  the  results  obtained  in 
practical  mill  work. 

Warm  solutions,  100°  Cold  solutions,  65° 

48  hr.  0.015  oz.  Au.  2.1  oz.  Ag.  0.02  oz.  Au.  3.8  oz.  Ag. 

60  hr.  0.01    oz.  Au.  1.4  oz.  Ag.  0.02  oz.  Au.  3.0  oz.  Ag. 

I  think  that  Mr.  Megraw's  assertion  that  the  addition  of  lead  and  lime 
in  Tonopah  practice  is  " principally  by  convenience"  is  a  little  broad. 
In  this  mill  at  least,  the  lime  content  is  very  closely  watched,  it  having 
been  proven,  not  only  by  laboratory  tests,  but  on  a  working  scale,  that 
extraction  is  very  erratic,  due  to  reprecipitation  when  either  too  high  or 
too  low  alkalinity  is  carried.  The  lime  is  maintained  between  1  Ib.  and 
1.5  Ib.  of  CaO  per  ton  of  solution. 

The  amount  of  lead  giving  best  result  is  at  all  times  somewhat  of  a 
conjecture,  but  in  order  that  it  may  not  be  understood  that  lead  is  added 
at  random  and  that  we  have  some  fixed  ideas,  even  though  they  may  be 
wrong,  the  practice  and  theory  followed  at  this  plant  are  here  stated. 

It  is  figured  that  sulphur,  alkaline  sulphides,  and  other  reducers  have 
a  retarding  action  and  are  detrimental  to  the  highest  dissolving  power 
of  the  solutions,  and  in  order  to  keep  alkaline  sulphides  under  control, 
lead  acetate  is  added  to  precipitate  the  lead  sulphide.  The  amount  used 
is  determined  by  experimental  tests  with  clean  solution.  Zinc  in  solution 
will  act  to  a  great  extent,  especially  in  hot  solution,  the  same  as  lead, 
forming  zinc  sulpkide,  but  is  not  as  amenable  to  the  combination  as  lead. 
If  an  excess  of  lead  is  present  the  zinc  will  tend  to  accumulate  in  the 
solutions,  and  then  the  amount  of  lead  is  reduced,  allowing  the  zinc  to 
take  care  of  the  alkaline  sulphides.  In  this  way  a  kind  of  a  balance  of 
zinc  contents  is  maintained. 

In  regard  to  tube-mill  lining,  14  tests,  under  varying  conditions  of 
feed  and  moisture,  conditions  in  both  mills  being  as  nearly  identical  as 
possible  at  all  times,  showed  an  average  of  5%  more  product  of  minus 
200-mesh  with  silex  than  with  the  smooth  iron  liner. 

In  regard  to  El  Oro  lining,  which  according  to  Mr.  Megraw's  Mexican 
experience  is  an  improvement  over  all  other  linings,  and  which  he  in- 
timates has  never  been  tried  at  Tonopah,  I  think  he  failed  to  go  thor- 
oughly into  what  had  been  tried  or  considered,  and  why,  in  the  opinion 
10 


138  DETAILS  OF  CYANIDE  PRACTICE 

of  the  operator,  the  practice  being  followed  is  best  suited  to  their  condi- 
tions. 

The  Montana-Tonopah  Mining  Co.  tried  El  Oro  liners,  finding  their 
life  to  be  between  seven  and  eight  months,  while  the  cost  of  the  lining 
installed  was  over  twice  that  of  silex.  At  this  plant  the  cost  of  silex 
lining  per  mill,  when  lining  seven  mills,  was  as  follows:  Labor  at  $4.50 
per  day,  $42.25;  cement,  28  sacks  at  $1.035,  $28.89;  silex,  six  tons  at 
$42.80,  $256.80,  or  a  total  of  $327.94.  To  Mar.  20,  these  mills  have 
been  in  operation  eight  months,  crushing  13,269  tons  per  mill,  or  an 
average  of  55  tons  per  day  with  the  following  average  discharge  screen 
analysis:  On  100-mesh,  1.33%;  on  150-mesh,  5.05%;  on  200-mesh, 
6.42%;  through  200-mesh,  87.20%.  This  is  an  average  of  36  tests  made 
during  eight  months. 

To  this  date  none  of  these  mills  has  had  to  be  relined,  but  it  is  figured 
that  the  first  mill  will  have  to  be  lined  in  about  two  weeks  and  the  entire 
installation  during  the  next  60  days.  Allowing  eight  months,  which 
is  very  good  life  for  this  style  of  liner,  the  cost  compares  more  than 
favorably  with  any  available  figures  of  El  Oro  liners.  In  "A  Textbook 
of  Rand  Metallurgical  Practice,"  Vol.  II,  p.  151,  the  following  statement 
is  made:  "  While  the  El  Oro  liner  has  a  longer  life  than  the  silex  liner, 
the  difference  is  not  great  enough  to  compensate  for  the  extra  cost." 

In  the  original  plans  for  our  Tonopah  mill,  rolls  were  seriously  con- 
sidered by  comparison  of  all  data  to  be  had  at  that  time  and  were  only 
abandoned  after  an  extended  visit  to  Mammoth  Copper  Co.'s  plant  at 
Kennett,  Calif.,  where  27,000  tons  of  Belmont  ore  were  sampled  as 
custom  ore  and  crushed  with  rolls.  The  excessive  wear  of  roll  shells 
and  total  cost  per  ton  resulting  in  the  eminently  conservative  installation 
of  stamps.  The  general  manager  of  one  of  the  late  large  cyanide  mill 
installations,  where  rolls  are  used,  told  the  writer  after  six  months' 
operation,  that  he  was  sorry  stamps  had  not  been  installed  instead  of  rolls. 

,    A.  H.  JONES. 
Tonopah,  Nev.,  Mar.  18,  1913. 


In  this  letter,  Mr.  Jones  adds  some  interesting  facts  to  the  discussion 
of  cyanide  problems  and  his  statements  will  be  accepted  as  authoritative 
on  Tonopah  practice.  Except  in  those  instances  where  he  has  misin- 
terpreted the  meaning  of  my  remarks,  we  are  substantially  in  accord. 

Regarding  crushing  in  water,  I  agree  with  Mr.  Jones'  reasoning. 
In  my  article,  the  matter  is  referred  to  in  a  discussive  way  and  not  as  a 
recommendation.  Reference  to  the  article  in  question  will  show  this. 

The  remarks  on  agitators  and  temperature  of  treatment  solutions  are 
interesting  additions  to  the  general  data  on  Tonopah  practice. 


PRACTICE  AT  TONOPAH  139 

Mr.  Jones'  objection  to  my  remarks  referring  to  addition  of  lime  and 
lead  acetate  is  justified  by  his  misinterpretation  of  the  text  of  my  article. 
" Principally  by  convenience"  referred  to  the  method  of  addition,  not 
to  the  quantity  of  the  chemicals  used. 

Regarding  tube-mill  lining,  it  seemed  worth  while  to  call  attention 
to  the  difference  between  practice  at  Tonopah  and  in  Mexico.  I  did  not 
attempt  to  convey  the  impression  either  that  the  El  Oro  lining  was  the 
best,  or  that  it  had  not  been  tried  at  Tonopah.  What  I  wished  to  say 
was  that  it  is  not  considered  the  best  in  Tonopah  and  is  not  in  use  there, 
although  it  has  been  proved  by  repeated  tests  to  be  best  suited  to  condi- 
tions in  some  other  districts. 

Stamps  versus  rolls  may  be  the  subject  of  interesting  and  lengthy 
discussion.  I  hope  that  Mr.  Jones  may  be  induced  to  publish  the  facts 
and  figures  which  led  to  the  decision  in  favor  of  stamps  at  the  Belmont 
mill.  Some  others  have  adopted  rolls  and  do  not  announce  any  regret. 

H.  A.  MEGRAW. 
New  York,  Mar.  24,  1913. 


Crushing  Tonopah  Ores 

The  following  data  relative  to  crushing  Tonopah,  Nev.,  ore  by  rolls 
may  be  of  interest  in  the  discussion  of  the  subject  invited  by  H.  A.  Megraw 
in  his  article,  recently  published  in  the  JOURNAL.  A  set  of  10X30-in. 
Allis-Chalmers  rolls  was  among  the  equipment  of  the  crushing  and 
sampling  department  of  the  Desert  Power  &  Mill  Co.  These  rolls  were 
in  operation  four  years,  from  December,  1906,  to  January,  1911,  and 
during  that  time  reduced  29,873  tons  of  ore,  which  was  5%  of  the  entire 
tonnage,  cut  out  by  a  Snyder  sampler.  The  roll  feed  was  the  same  as 
that  going  to  the  stamp  batteries,  varying  in  size  from  slime  to  pieces  of 
ore  2  in.  in  diameter,  this  size  depending  upon  the  condition  of  the  4D 
crushers.  The  maximum  size  of  the  roll  product  varied  between  J  and  f 
in.,  depending  upon  the  opening  between  faces  of  the  rolls.  However, 
usually  the  reduction  would  be  about  50%.  The  average  time  run  per 
day  was  5  hr.  for  a  total  of  1195  days  in  operation. 

The  power  is  estimated  at  10  hp.  and  on  that  basis  would  amount  to 
a  total  of  44,573  kw.-hr.,  which,  at  $0.0152  per  kw.-hr.,  gives  the  total 
power  cost  $677.52,  or  $0.0227  per  ton  of  ore  reduced  by  rolls.  It  is 
estimated  than  one-fourth  of  one  man's  time  at  $4.50  per  8-hr,  shift,  or 
$0.703  for  five  hours,  a  total  of  $840.09,  or  $0.0281  per  ton,  is  a  fair 
allowance  for  the  labor  of  operating  the  rolls  during  the  1195  days. 
Roll  shells  were  refaced  in  a  lathe  and  five  changes  were  made;  2J  sets 
were  worn  out  during  the  period  of  operation.  The  roll  shells  weighed 


140 


DETAILS  OF  CYANIDE  PRACTICE 


757  Ib.  each,  so  that  the  total  consumption  of  steel  amounted  to  4163.5  lb., 
or  0.14  lb.  per  ton  of  ore  reduced.  The  rolls  were  operated  at  103  r.p.m., 
which  gives  a  peripheral  speed  of  about  800  ft.  per  minute. 


REPAIR  COST  FOR  ROLL  OPERATION 

Labor  changing  rolls  @  $20 . 00,  per  set 

10  roll  shells  turned,  6  hr.,  each  @  69  cents  per  hr 

80  ft.  of  10-in.  belt  @  96  cents 

General  repairs  to  feeder,  belt  liners,  etc 

8  bearings  babbitted  @  $5 . 00  for  labor 

Cost  of  babbit  @  $10.00  each 

2f  sets  of  shells  @  $146. 62  per  set.  . 


Total 

Per  ton  of  ore  reduced 


$100.00 
41.40 
77.20 
100.00 
40.00 
80.00 

$403 . 20 

$841 . 80 
0.0282 


SUMMARY 


Total 

Per  ton 
reduced 

Repairs  and  upkeep  

$841  80 

$0  0282 

Labor  of  operating  
Power  used  

840.09 
677  52 

0.0281 
0  0227 

Total  

$2359.41 

$0.0790 

It  will  be  noticed  that  some  of  the*  above  figures  are  based  upon 
estimates,  but  it  is  believed  that  the  estimates  are  liberal.  I  have  had 
the  impression  that  rolls  can  be  used  to  advantage  in  the  milling  of 
Tonopah  ore  and  am  of  the  opinion  that  a  thorough  test  and  comparison 
would  show  a  saving  in  initial  cost  of  installation  as  well  as  in  operating 
expense,  and  would  welcome  the  publication  of  the  figures  obtained  by 
A.  H.  Jones  when  he  was  investigating  the  subject  of  roll  crushing  before 
stamps  were  decided  upon  for  the  new  Belmont  mill. 

A.  R.  PARSONS. 
Monrovia,  Calif.,  May  13,  1913. 


Calculation   of   Extraction 

In  the  third  article  on  " Silver  Cyanidation  at  Tonopah,"  by  Herbert 
A.  Megraw,  in  the  JOURNAL  for  Mar.  8,  1913,  there  is  expressed  some 
doubt  as  to  whether  the  method  of  calculating  extraction  is  better  than 
the  old  system  based  on  samples  of  the  mill  heads.  We  are  familiar 
with  the  trouble  between  the  millman  and  the  general  manager,  when 
the  head  samples  indicate  extremes  of  extraction  varying  from  70%  to 
120%,  but  it  is  my  opinion  that  the  fault  lies  not  in  the  mill,  but  in  the 
assay  office. 

When  treating  a  gold  ore  it  is  possible,  due  to  the  slight  mixing  the 


PRACTICE  AT  TONOPAH  141 

samples  receive  in  crushing  from  1J  in.  to  the  size  that  will  pass  a  60-mesh 
screen,  for  a  particle  of  gold  the  size  of  the  screen  opening  to  pass  in  excess 
into  the  final  assay  sample  or  to  be  retarded  and  thrown  away  with  the 
discard,  causing  an  appreciable  error  in  either  case. 

A  60-mesh  Tyler  screen  has  an  opening  of  0.221  mm.,  and  the  cube 
of  this  aperture  is  0.01079  cu.mm.  Assuming  the  gold  in  the  ore  to  be 
700  fine,  silver,  250,  and  base,  50  (sp.  gr.  7),  then  the  specific  gravity  of 
the  gold  as  it  goes  into  the  assay  will  be  16.61,  and  the  value  of  the 
same  per  ounce,  $14.62,  when  the  silver  has  a  value  of  $0.60  per  oz. 
The  weight  of  one  of  the  cubical  particles  of  gold  that  will  pass  a  60-mesh 
screen  would  be  0.01079X16.61,  or  0.1792  mg.  For  assay  29,166  mg.  is 
taken,  and  if  a  particle  of  gold  the  size  of  the  aperture  passes  in  excess 
into  the  final  assay  sample,  or  is  retarded  with  the  discard,  there  is 
0.1792X14.62,  or  a  value  of  $2.62  difference  from  the  true  assay,  repre- 
senting .26%  on  $10  ore. 

In  an  all-sliming  plant  the  screen  tests  will  usually  show  about  80% 
through  200  mesh,  19  to  20%  on  200  mesh,  and  1%  or  so  on  100  mesh. 
The  tailing  sample  is  certainly  more  intimately  mixed  in  its  passage 
through  launders,  agitators  and  filters,  than  would  be  economically 
possible  for  the  head  sample.  For  comparison  I  shall  assume  the 
cubical  size  of  the  screen  opening,  although  it  is  certain  that  with  effective 
treatment  the  metal  in  tailing  is  not  fully  exposed  to  the  dissolving 
effect  of  the  cyanide  solution,  but  exists  in  the  grains  of  quartz  or  sulphide 
in  quantities  much  smaller  than  the  inclosing  grains.  The  cubical 
opening  of  a  Tyler  200-mesh  screen  is  0.0004052  cu.m.,  and  its  weight, 
if  a  gold  cube,  0.00673  mg.,  with  a  value  of  $0.098,  80%  by  weight 
being  taken  as  through  200  mesh,  the  value  affecting  the  sample  is  $0.0784. 
The  cubical  opening  in  a  100-mesh  screen  is  0.002744  cu.mm.,  and  its 
value  is  $0.67.  This,  with  20%  of  the  sample  on  100  mesh,  gives  a  value 
which  may  affect  the  final  result  of  $0.13.  The  small  portion  on  100 
mesh,  all  of  which  will  pass  80  mesh,  cannot  account  for  a  difference  of 
more  than  $0.01.  The  sum  total  giving  a  value  of  $0.22,  which  would 
have  an  effect  of  only  2.2%  on  extraction. 

Having  shown  the  value  of  the  particles,  it  would  not  be  amiss  to 
show  the  number  of  sand, and  gold  particles  in  each  case,  arid  their 
proportions.  The  specific  gravity  of  tailing  being  about  2.6,  the  weight 
of  a  cubical  particle  that  will  pass  a  60-mesh  screen  is  0.01079X2.6,  or 
0.02805  mg.  In  one  assay  ton  there  are  1,039,809  possible  particles  of 
this  size.  In  ore  assaying  $10  and  bullion  worth  $14.62  per  oz.,  we  have 
0.684  mg.  in  one-assay  ton,  which  divided  by  0.1792,  gives  3.817  particles 
of  gold.  Having  3.817  particles  of  gold  that  will  pass  the  60-mesh 
screen,  there  will  be,  to  complete  the  assay  ton,  1,039,785  particles  of 
sand. 


142  DETAILS  OF  CYANIDE  PRACTICE 

In  the  case  of  the  slime  the  value  is  assumed  proportional  to  the 
screen  tests  by  percentage.  With  a  total  value  of  $0.50,  the  value  of 
the  minus  200-mesh  material  is  $0.40,  and  the  weight  25,332.8  mg.  The 
weight  of  a  gold  cube  equal  in  size  to  the  aperture  in  a  200-mesh  screen  is 
0.00673  mg.,  and  $0.40  represents  0.02736  mg.  Therefore,  there  are  4,065 
particles  of  gold,  and,  consequently,  25,327.735  mg.,  exclusive  of  the  gold. 
Then  25,327.735  divided  by  0.0004052X2.6  gives  24,083,772  particels, 
exclusive  of  the  gold. 

With  19%  on  200  mesh,  the  weight  is  5541.5  mg.,  and  the  weight  of 
the  gold  cube  the  size  of  the  screen  opening  is  0.04558  mg.,  giving  actually 
0.1462  particles  of  gold  in  this  part  of  the  sample;  5541.54  mg.,  minus 
0.0065  mg.,  gives  5541.5335  mg.,  which,  divided  by  0.002744X2.6  gives 
776,734  particles,  exclusive  of  the  gold. 

On  100  mesh  there  is  1%  containing  $0.005,  and  weighing  0.00034 
mg.,  the  cubical  opening  of  an  80-mesh  screen  is  0.005178  cu.mm.,  and 
the  weight  of  the  same  cube  of  gold  is  0.0756  mg.  Therefore,  0.00034 
divided  by  0.0756  gives  0.0045  particles  of  gold,  which  may  remain  on 
100  mesh.  There  are  then  291.66  mg.  minus  0.00034  mg.,  or  291.6596 
mg.  of  sand,  and  291.6596  divided  by  0.005178X2.6,  or  21,664  particles 
besides  those  of  the  gold. 

To  sum  up  these  facts,  we  have,  in  heads,  3.817  particles  of  gold, 
and  1,039,785  particles  of  sand,  or  a  ratio  of  1  to  271,800,  and  in  tailing 
4.2115  particles  of  gold,  and  24,882,170  particles  of  sand,  or  a  ratio  of  1 
to  5,908,000.  From  the  ratio  of  gold  particles  to  sand  •  particles,  the 
chances  of  having  an  error  in  the  head  sample  as  compared  with  the  tail 
sample  is  about  21 J  times  as  great. 

Of  the  usual  head  sample  only  one  is  taken,  while  with  tailing  a  sample 
is  generally  taken  from  each  cycle,  at  least  four  samples  a  day,  and  any 
discrepancy  in  one  is  partly  compensated  by  the  assayer  when  he  takes 
the  mean  of  the  four. 

Tons  stamped,  and  tons  discharged  may  not  be  the  same,  but  under 
proper  operating  conditions  the  results  over  a  period  of  12  months  will 
not  indicate  enough  of  a  difference  to  justify  a  suspense  account. 

Providing  there  are  no  mechanical  wastes  of  slime  or  solution,  except 
in  the  regular  filter  discharges,  then  the  value  of  bullion,  plus  the  value  of 
tailing  discharged,  divided  by  the  number  of  tons  treated,  should  give  a 
closer  true  value  of  each  ton  of  ore  than  the  older  methods. 

A.  G.  CADOGAN. 
Puntarenas,  Costa  Rica,  Apr.  25,  1913. 


In  the  JOURNAL  of  Mar.  8,  1913,  H.  A.  Megraw,  in  his  article,  "  Silver 
Cyanidation  at  Tonopah — III,"  states  that  in  most  of  the  mills  there,  the 


PRACTICE  AT  TONOPAH  143 

calculation  of  extraction  is  performed  by  using  the  content  of  bullion 
produced  plus  the  content  of  tailing  discharged  as  the  content  of  head 
samples.  This,  no  doubt,  is  excellent  from  the  point  of  view  of  the 
millman  in  charge  of  the  plant  and  doubtless  saves  him  much  worry, 
but  from  the  point  of  view  of  accuracy  it  leaves  something  to  be  desired. 

There  may  be  serious  errors  constantly  occurring,  which,  with  the 
above-mentioned  method  of  calculation,  would  never  be  suspected. 
Quite  considerable  quantities  of  bullion  might  be  stolen  and  no  one  be 
any  the  wiser.  Moreover,  there  is  no  way  of  finding  out  if  there  is  any 
serious  unnoticed  leakage  in  the  plant. 

No  doubt  the  assayer  can  be  checked  with  duplicate  samples,  but  the 
greatest  test  on  his  work  and  on  the  tonnage  calculation  is  whether  the 
output  of  bullion  tallies  with  the  theoretical  quantity  required.  With 
careful  work  there  should  be  no  reason  why  the  bullion  output  should 
not  check  within  reasonable  limits.  This  gives  satisfaction  to  every  one 
concerned  and  then,  should  there  be  any  serious  discrepancies,  the  cause 
will  probably  be  noticed  almost  at  once,  whereas  with  an  adjusted  method, 
such  as  that  used  at  Tonopah,  there  can  only  be  an  accumulation  of 
error. 

The  fact  of  crushing  in  solution  complicates  the  mill-head  sample 
slightly,  owing  to  metal  being  dissolved  while  in  the  mortar  box,  and  also 
by  the  bullion  content  of  the  cyanide  solution  in  circulation.  However, 
a  fair  sample  of  the  mill-head  can  be  obtained  from  the  pulp  on  leaving 
the  mortar  boxes.  If  this  is  assayed,  and  also  the  solution  before  and 
after  it  comes  in  contact  with  the  ore,  the  proportion  of  solution  to  ore 
being  known  and  its  bullion  content  allowed  for,  the  original  content  of 
the  ore  can  easily  be  calculated.  The  pulp,  of  course,  will  have  to  b 
washed  before  assaying. 

Extractions  varying  between  the  limits  of  60%  and  120%  are  cer- 
tainly annoying,  but  this  sort  of  thing  is  almost  inevitable.  However, 
these  variations  only  happen  from  month  to  month  and  the  important 
matter  is  what  the  annual  result  is  going  to  be;  the  monthly  reports  are 
only  to  see  how  the  production  is  progressing.  It  stands  to  reason  if 
for  several  months  an  unusual  quantity  of  bullion  is  taken  from  the  zinc 
boxes  and  extra  zinc  acided,  which  usually  happens  at  the  end  of  a 
financial  year  in  order  to  balance  accounts,  that  the  actual  production 
for  those  months  will  be  in  excess  of  the  theoretical. 

Later,  owing  to  less  short  zinc  being  present  in  the  boxes,  the  bullion 
distributes  itself  over  a  larger  proportion  of  long  zinc  than  usual,  with 
the  result  that  not  so  much  bullion  is  cleaned  up  and  there  is  an  apparent 
shortage.  Conversely  if  there  is  an  apparent  shortage  of,  say,  20%  on  a 
90%  extraction,  if  calculations  are  correct  and  there  is  no  other  source 
of  error,  there  is  no  reason  why  a  month  or  two  later,  taking  the  extraction 


144  DETAILS  OF  CYANIDE  PRACTICE 

as  being  90%,  there  should  not  be  an  actual  extraction  of  110%,  making 
up  for  the  original  20%  left  in  the  boxes.  About  the  middle  of  the 
financial  year,  the  boxes  would  be  in  normal  condition  and  one  would 
expect  the  theoretical  and  actual  extractions  to  be  about  the  same. 

•  Taking  the  year  1912  at  the  Mijnbouw  Maatschappij  Ketahoen,  the 
cyanide  plant  treated  47,597  tons  of  ore  with  a  total  content  of  7160.7  oz. 
gold  for  a  theeortical  extraction  of  88.7%,  calling  for  6350.55  oz.  The 
actual  gold  recovered  was  6325.22  oz.,  or  an  actual  extraction  of  88.3%, 
the  difference  being  25.33  oz.  The  silver  called  for  was  19,315.46  oz., 
whereas  19,224.46  oz.  were  recovered,  a  shortage  of  91  oz. 

Monthly  actual  extractions  varied  between  59.5%  and  123.3%. 
This  has  also  been  the  case  for  the  years  1909,  1910  and  1911  with 
equally  close  yearly  results. 

GEORGE  SIMPSON,  JR. 

Mijnbouw  Maatschappij  Ketahoen, 
Lebong  Soelit,  Benkoelen,  Sumatra^ 

May  1,  1913. 


An  article,  by  Herbert  A.  Megraw,  in  the  JOURNAL  of  Mar.  8,  on 
" Silver  Cyanidation  at  Tonopah — III,"  has  just  come  to  my  notice  and 
I  am  somewhat  surprised  to  learn  that  the  method  of  calculation  of 
extraction  referred  to  is  in  use  in  any  of  our  "  up-to-the-minute-in- 
efficiency" milling  plants,  and  I  am  inclined  to  at  once  agree  with  Mr. 
Megraw  that  the  question  of  such  method  should  be  discussed.  The 
estimation  of  mill-head  contents  by  adding  bullion  content  to  tailing 
content  and  dividing  by  tonnage,  is  not  a  method  worthy  of  a  metal- 
lurgist, and,  as  Mr.  Megraw  so  fittingly  puts  it,  is  certainly  l<  &  method  of 
solving  a  problem  by  means  of  which  any  answer  is  the  right  one." 

Such  methods  are  not,  I  believe,  in  general  use,  but  are  being  mostly 
used  by  operators  for  stock  companies.  There  are  two  classes  of  mining 
shareholders;  those  holding  shares  as  an  income-gaining  investment, 
and  those  holding  them  for  the  purpose  of  speculation.  Dividend- 
receiving  shareholders  do  not  exact  highest  efficiency.  They  are  glad 
to  get  a  dividend  instead  of  an  assessment  and  seldom  question  the 
methods.  Those  holding  shares  for  stock  operating  are  not  interested. 
If  the  mine  is  paying  its  way,  with  an  occasional  dividend,  declared  at 
the  right  time  to  stimulate  a  dull  market,  nothing  more  is  expected  or 
wanted.  However,  from  an  engineer's  standpoint,  or  from  a  business 
man's  standpoint,  the  system  leaves  much  to  be  desired. 

Not  only  is  the  system  referred  to  the  source  of  inaccurate  statements 
and  fictitious  results,  but  it  is  unscientific  and  unbusinesslike,  and  tends 
to  show  a  lack  of  moral  courage  on  the  part  of  the  well-paid  specialists 


PRACTICE  AT  TONOPAH  145 

in  charge  of  operations.  Advocates  of  the  system  will  claim  that  they 
are  continually  watching  results  and  working  toward  better  extractions, 
and  that  they  get  everything  possible  out  of  the  ore  anyway,  so  why 
undertake  additional  work  which  would  result  in  no  benefit  and  only 
confuse  the  records.  How  do  they  know  they  are  getting  everything 
possible  out  of  the  ore,  if  they  do  not  know  what  the  ore  contains  to  start 
with?  They  tell  by  the  tailing  assays;  if  these  are  low  they  assume  results 
to  be  correct.  How  do  they  know  that  the  bullion  recovered  is  the  true 
difference  between  head  and  tail  content?  Tailing  content  may  be  low 
and  still  the  bullion  produced  may  not  represent  what  should  have  been 
recovered  from  the  ore,  or  perhaps  what  really  was  extracted,  Losses 
may  occur  through  carelessness,  faulty  treatment,  or  dishonesty.  These 
losses,  which  may  amount  to  much,  are  entirely  unguarded  under  such  a 
system. 

If  an  operator  is  afraid  of  his  mill  head,  why  is  he  not  afraid  of  his  mill 
tailing?  He  probably  is  not  afraid  of  either  alone,  but  does  not  care  to 
meet  both  together,  particularly  when  he  is  standing  between  them  with 
the  "  brick."  There  is  no  reason  why  the  tailing  sample  or  assay  should 
be  more  reliable  than  the  head  sample  or  assay.  In  many  cases,  it  is 
much  less  reliable.  Who  ever  heard  of  a  correct  tailing  sample  during  the 
time  anything  was  going  wrong  in  the  mill,  especially  when  there  was  no 
record  of  heads?  How  can  tonnage  represented  by  tailing  samples  be  as 
accurately  established  as  tonnage  represented  by  head  samples?  In 
cases  where  sand  and  slime  are  treated  separately,  the  calculation  of 
proportionate  tonnage  increases  such  inaccuracy. 

When  the  head  content  is  known,  an  efficient  system  of  solution 
sampling  will  check  both  bullion  and  tailing,  but  when  the  head  is  un- 
known, this  will  only  check  the  bullion.  It  is  true  that  bullion  plus  tailing 
content  is  equal  to  the  head  content  if  no  losses  or  errors  occur,  but  they 
do  occur,  and  hence  the  formula  instead  of  being  B  +  T  =  H,  should 
be  written  B  +  T  +  L  =  H.  But  H  and  L  being  both  unknown 
quantities,  "any  answer  is  the  right  one." 

By  having  the  head  assay,  precipitation  record  from  solution  assays, 
tailing  assays  and  correct  tonnage  record,  results  are  positive  and  any 
discrepancy  represents  loss  or  error,  the  two  most  important  things  the 
metallurgist  is  paid  to  guard  against.  He  is  not  necessarily  careless  or 
incompetent  when  such  discrepancies  occur,  but  displays  his  ability  when 
he  finds  and  removes  the  causes  for  such  losses  or  errors.  Having  no 
head  assay,  it  requires  no  ability  on  the  part  of  the  metallurgist  to  keep 
his  records  free  from  discrepancies  except  that  he  must  be  able  to  add 
correctly. 

By  one  method  the  treatment  is  controlled  or  regulated  by  a  knowl- 
edge, before  treatment,  of  what  is  being  treated.  By  the  other  method 


146  DETAILS  OF  CYANIDE  PRACTICE 

the  treatment  is  controlled  or  regulated  by  a  knowledge,  after  treatment, 
of  what  was  treated.  By  one  method  the  operator  is  able  to  arrange  his 
treatment  so  as  to  secure  the  best  results,  by  the  other  he  makes  a  guess 
and  then,  after  treatment  is  finished,  he  determines  how  far  he  was  off 
and  makes  his  answer  right  by  fitting  his  original  problem  to  it.  He 
never  does  know  what  per  cent,  extraction  is  being  made  (a  $2  tailing 
may  be  the  result  off  better  per  cent,  extraction  than  a  $1  tailing),  not 
that  it  makes  any  material  difference,  as  the  actual  amount  of  bullion  is 
the  main  thing,  but  he  reports  his  percentages  just  the  same;  how? 

The  method  is  also  unfair  to  the  mine.  The  mill  keeps  its  record  clean 
and  all  contained  values  that  come  to  it  are  accounted  for  and  reported 
either  in  bullion  or  tailing,  but  the  mine  must  stand  its  own  losses  as 
well  as  those  of  the  mill.  The  mine  may  be  sending  $20  ore  to  the  mill, 
but  the  B  +  T  system  may  only  show  $18  as  mill  head,  which  is  all  the 
mill  will  acknowledge  receiving.  What  becomes  of  the  $2?  Where  is 
it  charged?  Who  loses  it?  It  flutters  about  unaccounted  for,  for  a 
time,  and  then  finally  settles  unowned  and  unnoticed  in  the  little  nest  of 
"  shrinkage  of  previously  estimated  ore  reserves,"  shown  in  no  report,  a 
most  convenient  hiding  place  for  inaccuracies,  losses  and  results  of  the 
inefficient  method  of  "  calculation  of  extractions."  If  a  careful  in- 
vestigation were  made  into  the  cause  of  discrepancies  in  mill  results,  on 
account  of  which  this  "  any-answer-is-the-right-one "  method  has  been 
adopted,  it  will  be  found  that  an  incorrect  tonnage  record  is  a  greater 
factor  in  the  matter  than  will  be  generally  conceded. 

A.  SIDNEY  ADDITON. 
San  Francisco,  Calif.,  June  14,  1913. 


The  custom  of  using  the  content  of  bullion  produced  plus  the  content 
of  tailing  discharged  as  the  value  of  the  feed  in  milling  operations,  as 
stated  by  Mr.  Megraw  in  the  JOURNAL  of  Mar.  8,  1913,  easily  accounts 
for  the  High  percentages  of  extraction  frequently  reported.  Occasionally 
more  reasonable  estimates1  may  be  found  in  the  reports  published  by 

A-B  When 

X100  C  A  =  Assay  value  of  feed; 


C  —  B  B  =  Assay  value  of  tailings ; 

—  =  %  extraction.  C  =  Assay  value  of  concentrates. 

A 

This  formula  would  give  correct  results  if  the  assays  of  feed  and  tailings  could  be 
relied  upon,  but  with  an  assay  of  settled  tailings,  i.e.,  tailings  minus  float,  the  extrac- 
tion percentages  so  obtained  may  be  more  than  18%  above  the  actual  extraction  and 

JA  formula  much  used  for  calculating  the  extraction,  and  which  is  stated  to  be 
convenient  for  approximate  estimations  when  it  is  impracticable  to  weigh  the  prod- 
ucts, is  the  following: 


PRACTICE  AT  TONOPAH  147 

it  is  therefore  undesirable  to  make  use  of  this  formula.  Ashcroft  "The  Flotation 
Process,"  "Trans.,"  I.  M.  M.,  1912,  and  "Eng.  and  Min.  Journ.,"  Dec.  7,  1912,  p. 
1085. 

large  mining  companies  working  low-grade  ores,  as  such  companies 
generally  make  use  of  sampling  devices  and  employ  their  own  assayers, 
but  no  calculation  into  which  the  assay  value  of  the  tailing  enters  can  be 
of  value  because,  under  working  conditions,  it  is  impossible  to  ascertain 
directly  the  real  value  of  the  tailing. 

From  the  moment  concentration  commences  the  water  made  use  of 
carries  off  mineral  of  value,  and  this  continues  all  through  the  process. 
The  amount  of  floating  mineral  will  vary  with  the  fineness  of  division  of 
the  particles,  degree  of  concentration,  amount  of  water  used,  and  in- 
clination of  the  tables,  or  with  the  rapidity  of  concentration.  The 
effect  of  these  circumstances  is  to  cause  the  settled  tailing  to  be  invariably 
of  lower  assay  value  than  if  there  were  no  float. 

The  subject  of  extraction  percentages  is  one  which  occupied  my 
attention  for  several  years,  owing  to  my  having  been  engaged  by  a 
mining  company  to  carry  out  investigations  with  a  view  to  ascertaining 
the  amount  and  manner  of  loss  in  the  wet  concentration  of  silver  ores. 
No  trouble  or  expense  was  to  be  spared  and  ample  opportunity  was 
afforded  to  make  experiments  on  whatever  scale  seemed  advisable. 

To  estimate  the  extraction  it  was  clearly  necessary  to  know  either  the 
correct  value  of  the  mill  feed  or  of  the  tailing.  To  obtain  the  tailing  value 
was  difficult  on  account  of  the  constant  loss  in  the  overflow  and  the 
impossibility  of  collecting  the  whole  of  the  material  under  ordinary 
circumstances  of  working,  and  it  was  therefore  decided  to  depend  on  the 
assay  value  of  the  feed. 

During  the  tests,  samples  of  the  settled  tailing  were  taken  and 
assayed,  but  in  no  instance  did  their  value  agree  with  the  theoretical 
value  obtained  by  difference.  Their  value  was  invariably  lower  and 
the  difference  greater  with  higher  degrees  of  concentration.  At  first,  it 
was  supposed  that  some  error  had  been  made  in  sampling  or  assaying, 
and  check  samples  were  taken  and  assayed,  but  these  only  confirmed  the 
low  value  of  the  settled  tailing.  In  the  opinion  of  the  operators,  the 
water  only  carried  off  the  more  soluble  portion  of  the  waste.  It  seemed, 
however,  that  the  mineral  accompanying  this  might  account  for  the  low 
value  of  the  settled  tailing.  After  this,  a  number  of  tests  were  made  to 
ascertain  the  extraction  in  jigging,  in  sand  treatment,  and  in  concen- 
trating slime,  by  concentrating  in  the  usual  manner  about  10  tons  of  ore 
in  the  same  condition  as  fed  to  the  different  machines,  it  being  possible 
in  concentrating  a  moderate  quantity  of  ore  to  collect  the  entire  tailing  in 
tanks.  As  a  result  of  many  tests  it  appeared  that  the  percentage  of 
floating  material  was  proportional  to  the  fineness  of  the  particles  treated 


148  DETAILS  OF  CYANIDE  PRACTICE 

and  that  its  value  was  more  than  twice  the  theoretical  assay  value  of  the 
tailing.  Thus,  the  discrepancy  between  the  theoretical  and  actual  assay 
value  of  settled  tailing  was  accounted  for. 

The  tests  referred  to  were  made  with  the  same  care  in  weighing, 
sampling  and  assaying  the  ore  and  products  of  concentration  as  would 
be  exercised  by  a  smelter  purchasing  ore,  and  were  under  my  constant 
personal  inspection,  the  object  being  to  arrive  at  the  actual  rate  of  loss 
per  degree  of  concentration  of  value  in  each  of  the  three  operations 
mentioned.  The  assumption  was  that  with  this  data  it  would  be  possible 
to  calculate  with  precision  the  probable  results  to  be  obtained  in  con- 
centrating ores  of  different  assay  value  to  different  degrees  of  value, 
by  the  methods  of  concentration  made  use  of  at  the  establishment  under 
consideration,  without  the  necessity  of  weighing  the  feed  or  the  tailing. 

Loss  in  concentration  appears  to  arise  from :  First,  the  unavoidable 
loss  characteristic  of  the  particular  ore  concentrated,  arising  from  the 
degree  of  natural  concentration  of  the  value  in  the  gangue,  its  combina- 
tion with  various  sulphides,  and  the  nature  of  these;  and  second,  the 
degree  of  division  of  the  particles,  which  is  the  main  cause  of  loss. 

Some  minerals  are  more  friable  than  others,  but  the  percentage  of 
mineral  to  gangue  is  so  small  that  it  would  not  materially  affect  the  general 
friability  of  the  mass,  and  it  may  be  assumed  that  ore  of  different  minerals 
will  crush  in  a  similar  manner  and  that  the  effect  of  division  would  be 
similar,  although  the  float  slime  might  be  of  higher  value  with  friable 
ore.  Hence,  the  weight  of  float  arising  from  a  certain  degree  of  crushing 
an  ore  may  be  taken  to  be  a  measure  of  the  percentage  of  float  in  crushing 
any  ore  to  a  similar  degree,  and  this  appears  to  be  confirmed  by  the  few 
examples  which  it  has  been  possible  to  obtain. 

The  accompanying  scale  of  extraction  percentages  obtained  in 
concentrating  a  silver  ore,  crushed  to  pass  a  30-mesh  screen,  aperture 
0.0166  in.,  will  illustrate  the  application  of  the  results  of  some  of  the 
tests  made.  The  silver  was  associated  mainly  with  iron  pyrites  and  a 
small  amount  of  blende  and  occasional  small  quantities  of  galena,  and 
occurred  as  pyrargyrite  and  in  combination  with  copper  (fahlerz). 
The  matrix  was  quartz  of  average  hardness. 

The  extraction  by  formula  based  on  the  assay  value  of  the  settled 
tailing  was  4.2669%  at  2  :  1  concentration,  and  18.3545%  at  12 :  1  con- 
centration, in  excess  of  the  actual  extraction. 

It  will  be  observed  that*  the  difference  in  the  assay  value  of  the 
settled  tailing  and  the  total  tailing  is  so  small  in  low  degrees  of  con- 
centration that,  with  ores  of  little  value,  it  might  escape  attention 
unless  great  care  were  taken  in  sampling  and  assaying. 

The  method  adopted  for  finding  the  percentages  of  extraction  and 
loss  incurred,  weight  and  value  of  concentrates  and  tailings,  for  any 


PRACTICE  AT  TONOPAH 


149 


degree  of  concentration  or  value  of  feed,  according  to  results  obtained  in 
carefully  made  tests,  was  as  follows: 


ii 


-o 


K)  «O  U3  u 


00    00    00    X    X 


(N     i-H     1-H     i-H 


•  Degree 
concentrati 
values 


150 


DETAILS  OF  CYANIDE  PRACTICE 


The  total  possible  loss  being  100%,  the  loss  due  to  different  degrees 
of  concentration  of  the  values  will  be  proportionate  percentages  of  this 
loss  according  to  the  degree  of  concentration: 

PERCENTAGE  OF  LOSS  IN  VARIOUS  DEGREES  OF    CONCENTRATION 
Degree  =  %  of  total  loss  Degree  =  %  of  total  loss 


0=0 

2=50.0 

3=62.5 

4  =  75. 

5=78.125 

6=81.25 

7=84.375 

8=87.5 


9  =  88.28 
10  =  89.06 
11=89.84 
12  =  90.625 
13=91.41 
14  =  92.19 
15=92.97 
16  =  93.75 


To  find  the  total  loss,  first  divide  the  assay  of  the  concentrate  by  the 
assay  of  the  feed  to  find  the  degree  of  concentration,  then  by  means  of  the 
table  find  the  relative  percentage  and,  from  this,  the  total  loss.  When  the 
total  loss  is  found  by  means  of  a  test,  the  loss  in  concentrating  similar 
ore  of  any  assay  value,  and  in  a  similar  state  of  division,  may  be  found,  as 
it  will  correspond  to  the  percentage  of  the  total  loss  for  that  degree. 
In  this  manner  the  percentage  weight  of  tailing  and  concentrate  may  be 
found  and  the  actual  value  of  the  tailing. 

The  total  loss  may  exceed  100%  when  the  rate  of  loss  is  great,  due  to 
water  used  in  concentration  being  excessive — or  there  is  a  reversal  of 
concentration. 

A  knowledge  of  what  the  actual  value  of  the  tailing  may  be  at  different 
degrees  of  concentration  is  of  great  importance,  as  concentration  may  be 
carried  to  such  an  extent  as  to  become  negative,  that  is  to  say,  the  discard 
will  be  of  greater  value  than  the  feed,  owing  to  the  tailing  increasing  in 
value  in  proportion  to  the  degree  of  concentration.  The  first  tailing 
discarded,  up  to  a  concentration  of  2  :  1,  is  the  largest  in  amount  and  the 
least  valuable.  After  this  it  becomes  richer  and  less  in  quantity;  this, 
however,  is  liable  to  pass  unnoticed,  as  the  amount  being  small  in  com- 
parison with  the  first  tailing,  the  average  value  of  the  whole  of  the  tailing 
is  not  greatly  increased.  The  following  example  of  concentrating  cop- 
per ore,  sand  and  slime,  with  bubbles  will  illustrate  this: 


Degree 
of 
concentration 

Tons 
concentrate 

% 

Tons 
tailing 

% 

Extraction 

% 

Loss 

% 

Average  value 
of  tailing 
units 

2 
4 
8 

22.56 
4.42 
0.495 

77.44 
95.58 
99.505 

45.12 
17.68 
3.96 

54.88 
82.32 
96.04 

7.0868 
8.6127 
9.6518 

The  unit  has  been  taken  as  10  for  feed. 


PRACTICE  AT  TONOPAH  151 

In  this  scale  of  concentration  the  total  loss  found  by  experiment  was 
109.76%,  so  that  there  was  a  reversal  of  concentration  at  some  point. 
Concentrating  22.56  tons  to  4  :  1  would  produce  4.42  tons  concentrate 
with  18.14  tons  tailing  at  15.127  units,  but  concentrating  to  2.5  deg.  the 
tailing  would  be  worth  9.463  units,  the  limit  to  which  concentration  could 
be  carried  without  reversal. 

In  the  vanner  treatment,  the  total  possible  loss,  found  by  experiment 
was  106.66,  about  the  same  as  with  the  buddies;  consequently  it  was  not 
possible  to  concentrate  beyond  2.5  degrees  ih  either  case.  The  con- 
centration, however,  was  carried  to  4.9471  degrees  by  vanners  and 
2.6277  degrees  by  buddies.  The  combined  concentration  was,  by  jigs, 
12.734  degrees,  extraction,  57.095%;  by  vanners,  4.9471  degrees,  extrac- 
tion, 17.589%;  and  by  buddies,  2.6277  degrees,  extraction,  36.545%; 
and  combined,  8.4692  degrees,  extraction,  69.575%.  Assuming  the  data 
given  to  be  correct,  the  tailing  would  have  an  average  content  per  ton, 
0.70443%  copper,  of  which,  according  to  the  loss  in  float  resulting  in 
tests  of  silver  ores,  35%  of  the  weight  of  tailing  would  have  been  carried 
off  with  57%  of  the  calculated  contents  of  same,  and  the  value  of  the 
settled  tailing  would  have  become  reduced  to  0.46594%  copper,  equal  to 
a  reduction  of  0.23849%  in  the  percentage  value. 

Calculating  the  percentage  of  extraction  by  the  formula,  with  value 
of  settled  tailing  0.4659%,  an  extraction  of  80.10%  would  be  obtained  or 
15.127%  in  excess  of  the  actual  combined  extraction.  The  excess,  how- 
ever, would  be  much  more  in  calculating  separate  recoveries  by  vanners  or 
buddies. 

The  relative  extraction  by  the  different  machines  at  the  same  degree  of 
concentration  was:  Jig,  76.48%;  vanner,  46.67;  buddle,  45.12%.  This 
must  not  be  considered  as  a  measure  of  efficiency  but,  if  anything,  a 
measure  of  the  fineness  of  division  of  the  particles.  The  float  percentage 
on  the  tailing  was:  Jig,  10  to  15%;  sand  tables,  30%,  due  to  a  portion 
being  reground;  buddies,  45%,  due  to  nearly  all  being  reground. 

A  single  test  was  not  found  to  be  sufficient  as  a  test  of  the  recovery  or 
of  the  amount  of  float  occurring  in  the  operations  of  a  mill,  because,  in 
working,  buddle  heads  are  sent  to  tables,  and  table  seconds  and  slime 
go  to  buddies,  and  these  combinations  alter  the  conditions.  It  was 
found  that  the  regrinding  greatly  increased  the  loss  and  this  without 
doubt  was  the  main  cause  of  lower  results  being  obtained  under  working 
conditions  than  in  single  tests.  The  main  loss  arises  from  float  whatever 
machine  is  used  and  there  appears  to  be  little  chance  of  overcoming  this 
difficulty.  The  great  loss  of  tin  slimes  in  Cornwall  is  evidence  of  this — 
but  it  may  be  possible  to  improve  extraction  by  paying  more  attention 
to  coarser  crushing  and  jigging.  It  is  not  sufficient  to  infer  from  observa- 
tion only  that  an  ore  requires  to  be  crushed  fine  to  obtain  the  best  results 


152  DETAILS  OF  CYANIDE  PRACTICE 

in  its  concentration  and  the  possibility  of  obtaining  better  results  from 
coarse  crushing  may  well  repay  the  trouble  of  carefully  ascertaining  the 
most  favorable  degree  of  crushing  in  each  case,  as  even  adjacent  veins 
vary  much  in  the  natural  concentration  of  their  values.  With  silver 
ores,  the  tailing  is  hardly  ever  of  lower  value  than  8s.  per  ton;  conse- 
quently, every  ton  of  waste  picked  out  by  hand  before  concentration 
results  in  a  considerable  saving,  as  it  raises  the  value  of  the  remainder, 
thereby  reducing  the  degree  of  concentration  required  and,  at  the  same 
time,  reducing  the  loss  in  treatment  of  the  whole. 

WILLIAM  S.  WELTON. 
London,  Eng.,  Apr.  8,  1913. 


CHAPTER  XIII 
THE  NEVADA  HILLS  MILL,  AT  FAIRVIEW 

The  plant  of  the  Nevada  Hills  Mining  Co.  is  situated  at  Fairview, 
about  45  miles  southeast  of  Fallon,  Nev.,  the  nearest  railroad  station. 
Due  to  the  lack  of  immediate  railroad  communication  the  entry  of 
supplies  is  slow  and  somewhat  expensive.  A  line  of  automobile  stages, 
however,  makes  passenger  and  mail  transportation  more  easy  and 
simple.  The  situation  of  the  plant,  so  far  removed  from  the  railroad 
point,  makes  the  obtaining  and  retaining  of  competent  labor  somewhat 
difficult.  The  fact  that  the  company  is  the  only  one  operating  actively 
in  the  district  does  not  help  matters  in  this  respect,  and  it  is  stated  that 
the  labor  question  is  one  of  the  important  ones  to  be  handled. 

Varied  Character  of  the  Ores. — The  ores  milled  at  the  Nevada  Hills 
mill  vary  in  both  character  and  hardness,  but  on  the  average  may  be 
said  to  be  somewhat  harder  than  the  usual  Nevada  silver  ores.  In 
character  they  vary  from  clean,  easily  treated  mineral  to  complex 
deposits,  which  give  some  trouble  in  beneficiation.  The  oxidized  ores 
contain  a  certain  quantity  of  manganese  and  are  generally  more  difficult 
to  cyanide  than  the  sulphides,  an  extraction  of  87%  being  usual  on  the 
former  and  on  the  latter  94%  or  more.  Calculating  on  a  basis  .of  the 
average  mill  run  an  extraction  of  92%  is  obtained.  The  mill  treats  from 
120  to  140  tons  per  day,  depending  on  the  character  of  the  rock.  The 
proportion  of  silver  is  about  as  100  :  1,  the  ordinary  mill  heads  averaging 
$20  to  $22  per  ton. 

The  mill  is  situated  at  the  shaft  head,  thus  obviating  transportation 
difficulties,  the  mine  skips  dumping  directly  into  a  bin  at  the  head  of  the 
mill.  The  mill  building  is  a  steel-frame  structure  covered  with  iron, 
substantial  and  well  built. 

Sampling  and  Weighing  Plant. — The  ore  coming  from  the  mine  is 
dumped  into  a  300-ton  bin  from  which  it  is  fed  to  a  Blake  crusher,  the 
product  of  which  passes  over  a  shaking  grizzly  into  an  elevator.  This 
elevator  lifts  the  rock  to  a  sorting  and  conveying  belt,  where  the  waste 
is  taken  out  after  the  whole  has  been  given  a  spraying  wash,  to  facilitate 
the  sorting  operation. 

The  original  design  of  the  mill  included  at  this  point  an  automatic 
weighing  device  and  sampling  plant,  but  both  of  these  have  now  been 
eliminated  from  the  operating  plan  and  the  passage  of  ore  is  as  above 
11  153 


154 


DETAILS  OF  CYANIDE  PRACTICE 


THE  NEVADA  HILLS  MILL,  AT  FAIRVIEW  155 

stated.  The  reasoning  leading  to  the  abandonment  of  these  steps  is 
that  the  expense  of  operating  them  was  more  than  any  advantages  gained 
from  them.  It  is  noteworthy  that  practically  the  same  condition  obtains 
at  the  mill  of  the  Goldfield  Consolidated,  at  Goldfield,  where  the  ex- 
tensive sampling  plant  originally  built  was  never  replaced  after  its 
destruction  by  fire. 

Whether  a  sampling  and  weighing  plant  is  worth  its  cost  of  installation 
is  a  question  which  seems  to  have  been  decided  in  the  negative  in  recent 
years,  although  in  former  times  such  refinements  were  considered  indica- 
tions of  approximate  perfection.  The  system  of  calculating  extraction 
percentages  by  using  the  sum  of  bullion  output  plus  tailing  content  as  the 
content  of  ore  entering  the  mill  has  rendered  sampling  unnecessary  and 
weights  can  be  approximated  in  some  other  more  economical  manner 
than  by  actually  passing  the  mill  run  over  scales.  In  speaking  of  the 
Tonopah  mills  I  have  already  referred  to  this  method  and  in  writing  on 
conditions  at  the  Black  Hills  I  mentioned  the  advisability  of  sampling 
and  weighing  plants  for  these  mills.  These  later  facts  brought  out 
by  the  investigation  of  further  plants  will  modify  the  conclusions  made 
earlier,  but  even  at  this  time  I  do  not  feel  altogether  convinced  that  the 
methods  so  universally  used  at  the  present  time  are  the  best  ones. 

Methods  of  Calculating  Extractions. — The  arguments  used  by  ad- 
herents of  the  latest  method  of  calculating  extractions  are  that  even  with 
elaborate  sampling  plants  it  is  difficult  to  obtain  an. accurate  sample; 
that  even  if  accurate  samples  are  secured,  they  are  liable  to  corruption 
through  the  inaccuracies  of  assaying,  the  fact  being  advanced  that  there 
is  always  a  certain  loss  in  assays  and  that  the  loss  on  the  higher-grade 
samples,  heads,  is  greater  than  on  the  lower-grade  samples,  tailing;  and 
that  the  varying  amount  of  metal  contained  in  mill  pulp  and  solutions 
makes  accurate  calculation  an  impossibility  in  any  case.  All  these  argu- 
ments, no  doubt,  have  weight,  although  it  does  seem  that  some  method 
of  calculating  a  check  could  be  devised. 

As  concerns  sampling  and  weighing,  I  am  convinced  that  by  using 
properly  designed  apparatus  in^the  hands  of  competent  operators  an 
accurate  sample  is  possible.  And  also  that  chemical  science  is  not  so 
limited  but  that  an  exact  estimation  of  the  contained  metal  in  any  sample 
can  be  made.  As  to  the  conditions  within  the  mills  themselves,  that  is  an 
entirely  different  matter  and  a  question  not  so  easily  solved.  In  the  older 
style  of  mill  where  sand  and  slime  are  treated  separately,  the  slime  going 
through  the  mill  in  three  or  four  days  while  the  sand  portion  remains  in 
process  of  treatment  as  much  as  15  days,  it  is  evidently  impossible  to 
calculate  extractions  over  a  short  period  of  time,  although  if  the  period 
be  long  enough  a  fair  approximation  of  accuracy  should  be  obtainable. 
In  the  newer  mills  treating  all  the  ore  as  one  product,  calculations  ought 


156  DETAILS  OF  CYANIDE  PRACTICE 

to  be  considerably  simplified.  In  its  present  condition  this  question  can 
hardly  be  considered  definitely  settled  and  I  believe  operators  ought  to  dis- 
cuss it  publicly  and  see  if  some  satisfactory  solution  cannot  be  advanced. 
Chemical  Consumption  and  Operating  Cost. — The  consumption  of 
material  is  about  as  follows:  Cyanide,  2.7  lb.;  lime,  9  lb.;  lead  acetate, 
0.8  lb.;  zinc,  1.5  lb.  per  ton;  pebbles,  3  lb.  per  ton  of  ore  milled.  The  cost 
of  operation  at  the  Nevada  Hills  mill  is  given  completely  in  the  accom- 
panying table: 

OPERATING  COSTS  AT  NEVADA  HILLS  MILL 

Per   ton 
Crushing:  ore     milled 

Labor $0.052 

Repairs 0.003 

Power 0.016  $0.071 

Stamping : 

Labor 0.106 

Repairs 0.024 

Power 0.117  0 . 247 

Concentrating : 

Labor 0.087 

Material 0.011 

Power 0.021  0.119 

Elevating  and  separating: 

Labor... 0.042 

Material 0.004 

Power v. 0.025  0.071 

Assaying  and  sampling: 

Labor.. 0.134 

Material 0.001 

Power 0.008  0.143 

Tube  milling: 

Labor 0.040 

Material 0. 104 

Power 0.149  0.293 

Settling  and  agitating: 

Labor 0.105 

Material 0.655 

Power 0.062  0.822 

Continuous  dficantation: 

Labor 0 . 020 

Material 0.004 

Power 0.008  0.032 

Filtering: 

Labor 0.096 

Material 0.028 

Power..  0.052  0.176 


THE  NEVADA  HILLS  MILL,  AT  FAIRVIEW  157 

OPERATING  COSTS  AT  NEVADA  HILLS  MILL— (Continued) 

Per  ton 
ore  milled 

Discharging : 

Labor 0.066 

Material 0.046 

Power 0.013  0.125 

Precipitation: 

Labor 0.057 

Material 0.191 

Power 0.032  0.280 

Refining: 

Labor 0.073 

Material. .  . '. 0.077 

Power 0.003  0.153 

Heating: 

Labor 0.004 

Fuel  (oil) 0.052  0.056 

Water 0.005 

Surface  equipment: 

Labor ' 0,007 

Material 0.003 

Power 0.004  0.014 

General  expense : 

Labor 0.015  0.015 

Total $2.622 

Note — Cost  based  on  milling  4030  tons  of  ore  in  30  days. 

The  ore  ready  for  the  mill  is  dumped  into  a  600-ton  mill  bin  back  of 
the  batteries,  into  which  it  is  fed  through  feeders  of  the  Challenge  type, 
but  driving  by  means  of  a  wire  rope  instead  of  the  usual  grip  used  on  them. 

There  are  20  stamps,  each  1250  Ib.  in  weight,  making  107  drops 
through  6J  in.  and  crushing  through  8-mesh  screen.  The  pulp  from  the 
stamps  is  delivered  into  two  cone  classifiers,  each  36  in.  diameter,  where 
it  is  divided  into  two  portions,  one  containing  the  coarser  sand  and  the 
other  the  finer  part  of  the  pulp. 

Classified  Concentration. — The  underflow,  or  sandy  portion,  from  the 
classifiers  is  passed  to  a  pulp  distributor,  whence  it  is  distributed  to  six 
No.  2  Deist er  concentrators,  the  overflow  of  the  cone  passing  around  this 
concentration  system  into  a  second  battery  of  two  hydraulic  cone 
classifiers. 

The  tailing  from  the  sand  concentrators  is  passed  to  Akins  classifiers. 
The  overflow  from  the  first  classifiers  passes  to  the  second  battery,  where 
the  underflow,  containing  the  heavier  part  of  the  .pulp,  goes  to  another 
pulp  distributor  and  thence  to  eight  No.  3  Deister  concentrators.  The 


158  DETAILS  OF  CYANIDE  PRACTICE 

overflow,  consisting  of  slime  ready  for  agitation  treatment,  goes  directly 
to  the  first  thickener,  as  will  be  mentioned. 

From  the  slime  concentrators,  the  pulp  with  the  concentrate  removed, 
flows  to  a  distributor  which  divides  it  in  two  and  delivers  to  two  48-in. 
Akins  classifiers,  these  being  of  the  well-known  interrupted  spiral  flight 
type,  the  overflowing  slime  joining  that  coming  from  the  second  battery 
of  hydraulic  classifiers,  while  the  sand  is  passed  to  another  distributor 
which  delivers  to  two  tube  mills,  each  5  X 18  ft.  The  reground  product 
from  these  mills  is  returned  to  the  second  battery  of  classifiers,  where  the 
slimed  portion  is  taken  out  and  the  remaining  pulp  follows  through  the 
concentration  and  mechanical  classification  process.  This  procedure  is 
shown  in  the  accompanying  flow  sheet. 

Continuous  Agitation. — The  slime  for  agitation  is  taken  first  to  one 
Dorr  thickener,  34X12  ft.,  where  it  is  thickened  from  about  6:1  to  1:1. 
Issuing  from  this  thickener,  the  pulp  goes  to  the  agitation  series,  before 
reaching  which,  however,  it  is  diluted  to  2  or  2J :  1  by  solution  made  up  to 
treatment  strength,  which  contains  5  Ib.  KCN  per  ton.  The  solution 
used  throughout  the  mill  averages  3  Ib.  per  ton,' all  crushing  being  in 
solution. 

There  are  nine  agitation  tanks  of  the  Pachuca  type,  each  12  ft.  diame- 
ter by  32  ft.  deep.  The  pulp  for  treatment  passes  through  six  of  these 
tanks  in  series,  the  flow  being  continuous.  From  these  tanks  it  is  taken 
to  a  Trent  agitator  tank,  of  the  same  size  as  the  thickener,  and  thence  to 
a  34Xl2-ft.  Dorr  thickener.  Here  the  pulp  is  thickened  again  to  about 
1 : 1,  and  subsequently  diluted  with  fresh  made-up  solution.  The  pulp 
then  passes  to  another  series  consisting  of  three  Pachuca  tanks,  of  the 
same  size  as  those  formerly  mentioned,  through  which  it  passes  con- 
tinuously. 

This  interrupted  agitation  series  is  especially  worthy  of  note,  as  it 
accomplishes  a  continuous  agitation  and  also  a  change  of  solution,  a 
condition  making  for  better  extraction  and  one  which  I  advocated  some 
years  ago  for  silver  ores  when  continuous  agitation  was  proposed  but  not 
generally  practised.1  The  advantage  of  a  change  of  solution  during  a 
long  agitation  period  is  generally  recognized  and  this  method  of  ac- 
complishing it  can  hardly  fail  to  produce  beneficial  results. 

Washing  by  Decantation. — From  the  second  agitation  series  the  pulp 
is  passed  to  a  tank  29.5  ft.  diameter  by  13.5  ft.  deep,  in  which  is  installed 
a  Dorr  agitating  mechanism.  In  passing  it  is  worth  while  to  mention  the 
working  of  this  machine.  It  has  two  agitating  arms  which  revolve  at 
4J  r.p.m.,  and  the  central  air-lift  pipe  is  12  in.  diameter.  This  air  lift 
delivers  the  pulp  to  two  arms  suspended  above  the  surface  of  the  pulp  in 
the  tank  and  which  revolve  in  unison  with  the  submerged  arms.  The 

1  "Continuous  Processes,"  Mex.  Min.  Journ.,  August,  1910. 


THE  NEVADA  HILLS  MILL,  AT  FAIRVIEW 


159 


Solution 


Skip 

I -500  Ton  Bin 
l-l2"x20"B)ake  Cr»sher 

/-Shaking  Grizzly 

Elevator 
SortingBelt 

1-600  Ton  Milt  Bin 
4-Suspended  Feeders 

20-1250 1 b.Stemps,6jr"drop,  I07drops 
8  MeshScreen 

2~36  Hydraulic  Cone  Classifiers 

I -36" Pulp  Distributor 

/  6-  Ho.  2  De/'ste  r  Concentrators 

Concentrate 


2-36" Hydraulic  Cone  Classifiers 
1-36  "Pulp  Distributor 

8-No.3  Deister  Concentrators 

entrate 

1-36  Pulp  Distributer 

2-48" 'Ak ins  Classifiers 
1-36  "Pulp  Distributor 
2-5x18' Tube  Mil  Is 

1-34x12  'Dorr  Thickener 

6~l2x32'Pachuca  Tanks  inSeries 
I- 34x12 'Trent 'Agitator  Tank 
l-34'x!2' Dorr  Thickener 
*3-l2x32' Pachuca  Tanks  in  Series 
l-2e£xlSj  Dorr  Agitator  Tank 
3-34x12' Dorr  Thickeners 

rrre/7  Solution 

2-12' Oliver  Filters 


Residue 

l~PregnantSolution  Tank- 

2-36f'~40  Frame  Clarifying  Presses 

2-2 inc  Dust  Feeders 

Pumps 
£ 
§.      2- Merrill  Precipitation  Presses 

f" 

*i) 

^      Barren  Solution  Tanks 


Bullion  Furnaces 
FLOW  SHEET  OF  NEVADA  HILLS  MILL. 


160  DETAILS  OF  CYANIDE  PRACTICE 

outside  arms  consist  of  a  launder  which  carries  the  pulp  from  the  air  lift 
and  drops  it  at  various  points  over  the  tank  surface,  an  arrangement  of 
slots  or  openings  allowing  an  equal  distribution.  It  is  stated  that  tests 
have  shown  that  the  pulp  within  this  agitator  maintains  its  homogeneity 
at  all  points  within  2%  when  tested  by  specific  gravity,  pulp  assay  or 
screen  test. 

From  this  agitator  the  pulp  passes  to  three  Dorr  thickeners,  operated 
as  a  continuous-decantation  system.  The  flow  sheet  illustrates  the 
.operation  of  this  series  which,  it  will  be  noticed,  takes  in  barren  solution 
at  the  final  thickener  and  passes  it  back  through  successive  tanks  until 
it  overflows  at  the  first  one,  enriching  itself  and  impoverishing  the  pulp. 
This  is  an  exemplification  of  the  proper  working  of  the  counter-current 
decantation  method,  of  which  I  have  already  spoken.  By  its  use  an 
effective  washing  of  the  solids  is  accomplished  and  also  a  lightening  of 
the  burden  upon  the  filtering  machine. 

From  the  final  thickener  of  the  series  the  pulp  is  taken  to  two  12-ft. 
Oliver  filters  where  final  dewatering  is  done.  A  water  wash  is  applied  to 
the  ascending  side  of  the  filter,  but,  none  on  the  descend-side,  as  economy 
of  water  is  necessary  and  by  this  means  the  discharged  pulp  contains  a 
minimum  of  moisture.  The  slime  is  discharged  upon  a  conveyor  belt, 
by  which  it  is  carried  clear  of  the  mill  and  deposited  on  the  tailing  pile. 
It  contains  an  average  of  27%  moisture.  At  this  mill,  in  distinction 
to  the  experience  of  the  Tonopah  mills,  no  advantage  is  found  from 
heating  the  solutions  above  normal. 

The  solution  to  be  precipitated  is  passed  through  clarifying  presses  and 
zinc  dust  is  used  to  precipitate  the  metals.  The  precipitate  is  partially 
dried,  fluxed  and  melted  in  oil-fired  furnaces. 

The  lime  used  in  treatment  is  added  as  an  emulsion  at  the  slime  col- 
lector, the  quantity  being  about  10  Ib.  per  ton  of  ore  milled. 


CHAPTER  XIV 
PRACTICE  AT  THE  NEVADA  WONDER  MILL 

The  mine  and  plant  of  the  Nevada  Wonder  Mining  Co.  is  situated  at 
Wonder,  Nev.,  at  a  distance  of  about  63  miles  from  Fallon,  which  is  its 
nearest  railroad  station.  Communication  is  by  means  of  wagons  for 
freight,  and  automobile  stage  for  passengers  and  mail.  As  has  already 
been  mentioned  in  the  case  of  the  Nevada  Hills  company  at  Fairview,  the 
isolated  situation  is  responsible  for  high  transportation  charges  and 
some  measure  of  labor  difficulties,  the  same  condition  applying  at 
Wonder  as  at  Fairview,  but  with  somewhat  more  force  due  to  the  greater 
distance  from  communication. 

The  ores  treated  at  Wonder  are  clear  quartz  of  medium  hardness  and 
offer  no  special  difficulties  in  crushing.  The  plant  treats  an  average  of 
about  110  tons  per  day  of  24  hours,  the  mill  run  assaying  about  18  oz. 
silver  and  0.25  oz.  gold. 

For  convenience  the  property  is  operated  by  two  companies,  the 
mining  being  carried  on  by  the  Nevada  Wonder  Mining  Co.  and  the 
milling  by  the  Churchill  Milling  Company. 

In  its  present  condition  the  mill  embodies  a  great  many  changes  from 
its  original  treatment  plan,  some  of  these  changes  having  necessitated 
ingenuity  in  installing  additional  apparatus  in  space  which  was  not 
designed  to  receive  it.  Such  changes  often  force  awkward  arrangements, 
but  in  this  mill  most  of  them  have  been  planned  and  instituted  without 
mechanically  embarrassing  results. 

Crushing  Practice. — The  ore  from  the  mine,  after  being  weighed,  is 
passed  through  a  10Xl6-in.  Blake  rock  crusher  which  delivers  to  an 
inclined  belt  conveyor,  the  latter  delivering  into  the  battery  bin.  The 
battery  bin  is  of  2550  cu.ft.  capacity  and  holds  about  120  tons  of  ore. 

From  the  bin  the  ore  moves  by  gravity  through  gates  into  suspended 
Challenge  feeders  delivering  into  the  mortars.  It  is  noted  by  the  op- 
erators that  the  one-stage  crushing  through  a  Blake-type  breaker  throws 
a  somewhat  heavy  burden  upon  the  stamps.  The  crusher  allows  a 
proportion  of  large  pieces  of  rock  to  get  through  into  the  battery  bins  and 
these  may  at  times  cause  difficulty  in  the  mortars.  Probably  a  better 
course  is  to  crush  in  two  stages,  insuring  a  uniform  feed  of  proper  size  for 
stamp  crushing. 

There  are  ten  stamps  of  1400  lb.,  each  dropping  96  times  per  min. 

161 


162 


DETAILS  OF  CYANIDE  PRACTICE 


PRACTICE  AT  THE  NEVADA  WONDER  MILL  163 

through  8  in.  The  mortars  are  of  the  narrow,  rapid-crushing  type  and 
are  equipped  with  screens  of  the  square-mesh  variety  having  an  aperture 
of  f  in.  These  stamps  are  relied  upon  in  this  instance  to  bear  the  heavy 
work  of  reduction,  and  the  pulp  from  them  is  passed  to  one  Monadnock 
Chilean  mill  for  the  second  stage.  This  mill  makes  28  r.p.m.  and  is  fitted 
with  " ton-cap"  screens  having  an  aperture  0.08  in.  wide. 

Comparison  of  Time  Loss  with  Stamps  and  Chilean  Mills. — It  is 
worth  while  at  this  point  to  consider  the  relative  time  lost  on  account  of 
repairs  to  batteries  and  the  Chilean  mill.  "One  of  the  strong  points  made 
by  objectors  to  Chilean  mills  is  the  time  required  to  make  repairs  to  them, 
the  claim  being  that  stamps  keep  on  working  all  the  time  without  any 
great  loss  due  to  renewals  and  repairs.  A  great  many  operators  fail  to 
take  into  consideration  the  fact  that  while  the  stamps  seem  to  be  work- 
ing steadily  and  making  a  great  deal  of  noise  about  it,  it  is  rare  in  a  mill 
containing  any  great  number  of  them  that  there  is  not  one  or  more  hung 
up  for  adjustment,  setting  tappets,  putting  on  a  dropped  shoe,  aligning, 
or  doing  any  one  of  a  number  of  operations  that  are  necessary  with  that 
class  of  machinery.  When  a  small  proportion  of  stamps  are  hung  up  it 
does  not  make  any  apparent  difference  in  operations,  for  the  mill  is  mak- 
ing just  about  as  much  noise  as  if  they  were  all  running,  but  it  must  be  con- 
sidered that  each  individual  stamp  is  relied  upon  to  crush  a  certain  defi- 
nite number  of  tons  of  ore  in  a  year  and  every  hour  one  stamp  is  out  of 
commission  has  its  effect  upon  the  total  product.  On  the  contrary,  if  a 
Chilean  mill  is  stopped  for  repair  the  whole  work  is  hung  up,  there  is  no 
noise  and  it  seems  necessary  to  .make  every  effort  to  get  it  running  again. 
The  effect  is  to  make  it  appear  that  the  Chilean  is  taking  much  more  time 
for  repairs  than  the  stamps,  whereas,  if  all  the  minor  delays  of  stamps  are 
added  up  they  will  usually  approximate  as  much  or  more  than  those  made 
necessary  by  the  Chilean  mill.  As  an  instance,  at  the  Wonder  mill  during 
a  period  of  a  year  the  time  lost  for  battery  repairs  amounted  to  7.70  days 
and  the  Chilean  mill,  3.66  days.  There  is  a  1QO%  difference  in  favor  of 
the  Chilean  mill.  And  in  this  case  it  is  to  be  remembered  that  the  feed 
delivered  to  the  Chilean  is  too  fine  to  insure  maximum  efficiency  or  econ- 
omy of  wearing  parts.  Personal  experience  has  been  that  the  Chilean  re- 
quires a  percentage  of  coarser  feed  to  do  its  best  work  and  I  believe  most 
operators  agree  with  that  theory  today. 

Tube-mill  Grinding. — From  the  Chilean  mill  the  pulp  passes  to  a 
duplex  Dorr  classifier  where  the  already  slimed  portion  is  taken  out  and 
the  sand  passed  to  a  tube  mill.  This  mill  is  5  X  22  ft.  and  makes  28  r.p.m. 
It  will  be  noted  that  this  mill  is  of  the  long  type,  due  to  the  fact  that  it  was 
installed  before  the  advantages  of  shorter  tubes  of  the  same  diameter  were 
recognized.  The  mill  contains  a  lining  of  the  El  Oro  type,  one  of  the  very 
few  of  these  linings  I  have  seen  in  use  in  the  West. 


164 


DETAILS  OF  CYANIDE  PRACTICE 


Classification  of  the  tube-mill  product  is  somewhat  involved  and  is  an 
illustration  of  one  of  the  changes  made  in  the  original  mill  design  already 
mentioned.  The  stream  from  the  mill  is  split  into  two  parts,  each  going 
to  a  separate  belt  elevator,  one  of  which  sends  its  portion  directly  back  to 
the  Dorr  classifier  and  the  other  delivering  to  a  5-ft.  classifying  cone. 
The  underflow  of  this  cone  is  returned  to  tube-mill  feed  and  the  overflow 
is  taken  to  a  7-ft.  classifying  cone  which  takes  out  the  coarser  part  as 
underflow  and  returns  it  to  the  elevator  leading  back  to  the  Dorr  classifier, 
and  passes  its  overflow,  slime,  directly  to  the  slime  collector.  The  accom- 
panying flow  sheet  shows  the  movement  of  the  pulp  and  solution. 

Continuous  Agitation  System. — Slime  is  collected  in  a  Dorr  thickener, 
24X14  ft.,  and  the  overflow  solution  passes  to  the  precipitation  depart- 
ment or  to  the  battery  tank.  The  thickened  pulp  is  agitated  intermit- 
tently in  four  Pachuca  tanks,  each  15  X45  ft.  The  pulp  receives  about  40 


COST  OF  MILLING  AT  WONDER,  NEVADA 

(Tons  milled,  25,186  —  Period  of  one  year.) 

|      Labor           Supplies           Power 

Total 

Superintendence  $3,686  .  65 
Crushing  and  conveying  1,651  .  14 
Stamps  3,173.41 
Chilean  mill                                           1  435  97 

800.40 
2,658.55 
1,722.62 
2,213.44 
476  .  65 
1,660.12 
1,161.52 

$3,686.65 
3,070.05 
7,409.94 
5,173.55 
5,357.39 
1,988.50 
24,158.04 
4,793.78 
8,031.74 
8,940.40 
632.50 
2,248.55 
3,126.93 
883  .  02 
1,100.90 
1,337.33 
7,014.76 
171.81 

618.51 
1,577.98 
2,014.96 
2,126.37 
566  .  84 
15,775.08 
953.25 
7,044.47 
5,736.84 
88.87 
1,120.53 
503  .  97 
508.91 
383  .  42 
617.79 
5,293.14 
32.13 

Tube  mill  1,017.58 
Elevating  and  separating  945.01 
Agitating  6,722  .  84 
Filtering  2,679.01 
Precipitating  987  .  27 
Refinery  ;      3,048  .  98 
Concentrating  543  .  63 
Assaying  |      1,086.10 
Surface  and  plant1  2,101.77 
General  374.11 
Storehouse  677  .  59 
Water  and  fire  line                  •               719  54 

154.58 

41.92 
521.19 

39.89 

Steam  heat..  1,721.62 
Traction  engine  139  .  68 

Total  direct  32,711  .90 
Indirect 
Office  expense  j      1,371  .  48 
Company  house  86  .  06 
Taxes  and  insurance  

44,963  .  06 

822  .  39 
105.48 

11,450.87 

89,125.84 

2,193.87 
191.54 
2,933.61 
267.54 
5,586.56 
3.538 
0.222 
3.760 

Legal  and  traveling  i  
Total  indirect  -.  .  .  |      1,547.54 
Direct  cost  per  ton  
Indirect  cost  per  ton. 

927.87 



Total  cost  per  ton  

1  Refers  to  building  repairs,  outside  lighting,  removal  of  debris,  etc. 


PRACTICE  AT  THE  NEVADA  WONDER  MILL  165 

hours' agitation  in  these  tanks  and  the  solution  used  is  heated  to  about  80°. 
Solution  used  for  treatment  contains  4.5  Ib.  KCN  and  4  Ib.  lime  per  ton. 
Due  to  the  fact  that  the  ore  is  clean  and  docile,  high  extractions  are 
the  rule,  the  average  over  the  past  year  being  94.49%  of  the  gold  and 
94.74  of  the  silver.  This  percentage  of  recovery  is  good,  especially  so  on 
the  silver,  due  to  the  raising  of  the  temperature  of  the  solutions.  It  has 
been  proved,  in  this  mill  that  the  extraction  falls  off  approximately  2% 
when  the  solution  temperature  is  allowed  to  fall  to  or  below  50°  F.  during 
treatment. 

Continuous  Filtration. — Pulp  from  the  Pachuca  agitation  tanks  is 
delivered  to  a  stock  tank,  28  X 14  ft.,  which  is  equipped  with  a  mechanical 
agitator,  and  is  diluted  by  the  addition  of  2  ft.  of  barren  solution,  and 
from  this  tank  it  is  taken  to  a  Dorr  thickener.  The  overflow  of  this 
thickener  goes  to  the  precipitation  department  and  the  underflow  of 
thickened  slime  is  returned  to  the  stock  tank  again.  By  this  means  the 
pulp  is  given  a  light  washing  and  the  content  of  the  stock  tank  is  main- 
tained at  a  proper  thickness  for  filtering. 

Oliver  niters  are  used,  and  the  pulp,  having  already  received  the  pre- 
liminary washing  mentioned,  is  filtered  continuously,  receiving  a  final 
wash  at  the  filter.  The  wash  water  applied  at  the  filter  is  led  through 
hoses  to  a  shaking  pipe  where  it  is  sprayed  on  the  cake  in  a  constantly  mov- 
ing stream.  By  this  means  the  delivery  of  the  wash  to  any  one  point  is 
avoided  and  a  better  distribution  is  accomplished.  The  discharge  from 
the  filter  is  said  to  contain  about  50%  moisture. 

In  this  mill,  as  in  many  others,  the  centrifugal  pump  has  proved  a 
more  economical  and  satisfactory  method  of  moving  slime  pulp  than  the 
triplex  slime  pumps. 

Solutions  from  the  Dorr  thickeners  and  from  the  filters  are  passed  to 
the  pregnant-solution  tank  from  which  the  precipitation  system  is  fed. 
There  are  six  zinc  boxes,  each  with  seven  compartments  30X27  in.  area. 
Only  five  of  these  compartments  are  used  for  precipitation  and  the  other 
two,  at  the  heads  of  the  boxes,  are  filled  with  excelsior  and  used  for  clarify- 
ing the  solutions.  Many  of  the  Nevada  mills  are  using  excelsior  for 
clarifying,  as  has  already  been  mentioned  in  the  case  of  the  Tonopah  mills, 
and  it  seems  to  do  the  work  in  a  satisfactory  manner. 

The  precipitate  cleaned  up  is  dried  in  a  pah  which  is  fitted  with  elec- 
trical means  for  heating,  and  is  then  fluxed  and  melted  in  Faber  du  Faur 
furnaces  burning  oil.  In  order  to  avoid  the  melting  of  short  zinc  which 
accumulates,  an  acid  tank  has  been  installed  and  this  material  finds  its 
way  into  the  regular  bullion  production. 

Material  Consumption  and  Cost. — Consumption  of  material  at  the 
Wonder  mill  is  of  cyanide,  1.6  Ib.  per  ton  of  ore  milled;  of  lime,  5  Ib.; 
of  acetate,  0.8  Ib.;  of  pebbles,  3  Ib.  and  of  zinc  1.8  Ib.  A  feature  of  in- 


166 


DETAILS  OF  CYANIDE  PRACTICE 


Storage  Tank  for 
/-~^  Barren  Solution 

V\\  BatterySof. 
Tank 


Optional 
return  to 
battery  tank 


I- Mine  Car  Scale 
I- 10x16" Blake  Crusher 
Inclined  Belt  Con  veyor 

ti&ton  Storage  Bin 

2-Suspended  Choi  lie  nge 
-^      IO-l400Ib.Stamps-        „    feeders 
Over  to      8"drop,  16  drops  j  Scree  n 
l-VBox 

I-  Trent-  Monadnock  Mill, 

28  r.p.m.0.08"Ton  CapScreen 

J- Duplex  Dorr  Classifier 


1-5x22' Gates  TubeMill,21r.p.m. 
ElOro  Lining 

2 -Belt  Elevators 
2-  Riffled  L  aunders 

l~5  Classifying  Cone 
I- 7  Classifying  Cone 
l-24xl4fDorr  Thickener 


4-~l5x45  Pachuca  Tanks 


!-2oxl4  Stock  Tank,  mechanical 
Agitator 

I- 28x10' Dorr  Thickener 


2'8x  II  Oliver  Filters 


Pregnant  Solution  Tank 


i-^  6-Zinc  Boxes,  /  'Compartments  50x27 


i 


Bar  re  n  Solution  S  ump 

LIJ  Electric  Drier 

C)  2-faber  au  Faur  Furnaces 

— >  Bull  ion 

FLOW  SHEET  OF  NEVADA  WONDER  MILL. 


PRACTICE  AT  THE  NEVADA  WONDER  MILL  167 

terest  is  that  the  cyanide,  lead  acetate  and  lime  required  in  treatment  are 
all  ground  up  together  in  a  small  Chilean  mill  and  added  to  the  pulp  flow 
between  the  Chilean  mill  and  the  Dorr  classifier.  This  is  shown  on  the 
flow  sheet. 

The  cost  of  milling  is  shown  in  the  accompanying  table,  but  it  is  to 
be  remembered  that  the  period  of  one  year  covered  by  these  figures 
included  115  days  lost  due  to  lack  of  power  and  other  less  important 
causes,  expenses  naturally  continuing  to  a  large  extent  during  that  time. 
These  expenses  falling  on  the  reduced  tonnage  show  a  cost  abnormally 
high  and  it  should  be  said  that  during  the  last  two  months  when  the  mill 
was  operating  at  its  maximum  rated  capacity  of  approximately  110  tons 
per  day,  the  expense  dropped  to  about  $2.75  per  ton. 

Considering  the  situation  of  the  mill  and  the  difficulties  under  which 
the  operators  have  been  working,  the  results  obtained  are  extremely 
creditable. 


CHAPTER  XV 
METHODS  AT  REPUBLIC,   WASHINGTON 

The  mining  camp  of  Republic  is  situated  in  the  northwest  section  of 
Ferry  County,  Wash.,  about  25  miles  south  of  the  Canadian  line.  In 
common  with  most  mining  camps  it  has  been  the  scene  of  intermittent 
activity  and  decline,  and  has  had  a  hard  time  to  demonstrate  its  value. 
The  district,  originally  a  part  of  the  Colville  Indian  reservation,  was 
opened  in  1896  and  its  history  since  that  time  has  been  a  checkered  one  of 
alternating  interest  and  indifference.  Lately,  however,  its  mines  have 
begun  to  be  developed  in  a  methodical  manner  which,  if  slow,  is  at  least 
more  satisfactory  and  certain  than  can  be  obtained  by  a  succession  of 
booms. 

Metallurgical  History  of  the  Camp. — Republic-  has  seen  a  series  of 
mills  built  within  her  boundaries,  none  of  which  seems  to  have  made  any 
remarkable  success.  The  first  one  was  built  for  the  Republic  Gold  Min- 
ing &  Milling  Co.,  and  made  use  of  the  Pelatan-Clerici  process.  Natu- 
rally, this  was  not  much  of  a  success,  the  saving  amounting  to  perhaps  50% 
of  the  contained .  silver  and  gold.  Then  followed  a  series  of  plants 
embodying  the  chlorination  process,  combined  amalgamation  and  cya- 
nide, the  Hendryx  cyanide  system,  and  finally  the  plant  constructed  by 
D.  C.  Jackling  for  the  Republic  Power  &  Cyanide  Co.,. built  in  1900. 
This  plant  operated  for  about  10  months  and  was  then  shut  down.  It 
was  patterned  after  the  old  Mercur  plant  in  Utah,  and  included  crushing 
by  means  of  rolls  and  Griffin  mills,  roasting  and  leaching.  Precipitation 
was  attempted  by  the  old  method  of  using  zinc  dust  in  tanks,  as  was  done 
at  Mercur.  This  mill  is  reported  to  have  saved  about  91  %  of  the  gold  in 
the  ore  but  only  about  15  %  of  the  silver. 

Due  to  the  many  methods  of  treatment  and  the  apparent  failure  of  all 
of  them  to  accomplish  any  commercially  successful  results,  the  camp 
received  a  bad  name  and  the  ores,  while  the  quantity  and  grade  were 
acknowledged,  were  thought  impossible  to  treat  by  any  means  sufficiently 
cheap  to  insure  profit.  As  a  matter  of  fact,  the  trouble  appears  to  have 
been  the  persistent  attempts  to  fit  a  process  which  had  proved  useful 
somewhere  else  to  the  ores  at  hand,  which  happened  to  be  not  in  the  least 
appropriate  for  it. 

There  seems  to  have  been  a  remarkable  lot  of  misinformation  pub- 
lished about  the  character  of  the  ores.  Some  experimenters  went  to  a 

168 


METHODS  AT  REPUBLIC,  WASHINGTON 


169 


12 


170  DETAILS  OF  CYANIDE  PRACTICE 

great  deal  of  trouble  to  prove  that  they  were  not  treatable  cheaply,  and 
apparently  satisfied  themselves  and  a  large  portion  of  the  mining  public. 
That  the  mineral  itself  does  not  justify  this  conclusion,  the  two  plants 
now  in  operation  prove. 

Description  of  the  Ore. — The  ore  is  described  as  being  a  fine  grained, 
close  textured  quartz,  but  there  is  also  some  white  quartz  which  is  not  so 
hard  and  which  is  easily  distinguished  from  the  blue,  flinty,  hard  material 
mentioned.  Ore  from  some  portions  of  the  camp  is  more  like  a  clay  and 
contains  an  extremely  large  amount  of  colloid  matter.  It  will  be  seen 
that  there  is  a  great  variety  of  ore,  but  that  at  present  worked  by  the  mills 
consists  largely  of  the  blue  quartz  and  white  quartz,  and  is,  in  general, 
hard. 

While  it  has  been  repeatedly  stated  that  this  ore  contains  elements 
which  make  it  rebellious  to  cyanide  treatment,  such  as  tellurium,  selen- 
ium and  copper,  repeated  tests  have  failed  to  detect  tellurium,  selenium 
exists  only  in  minute  traces  in  ores  of  milling  grade,  and  copper  only  in 
small  quantities.  Neither  of  these  last  two  elements  presents  any  insuper- 
able obstacle  to  cyanidation  as  selenium  compounds  are  usually  soluble 
and  exist  in  quantities  too  small  to  consume  excessive  amounts  of 
cyanide,  and  the  same  is  true  of  copper,  which  occurs  in  somewhat  larger 
quantities. 

It  is  undoubtedly  true  that  the  ores  did  not  prove  amenable  to  the 
processes  applied  to  them,  but  as  has  been  stated,  these  methods  were  not 
adapted  to  the  ores.  Later  methods  and  careful  experimentation  has, 
however,  opened  up  an  entirely  new  viewpoint. 

In  discussing  the  matter,  H.  W.  Newton,  superintendent  of  the  mill  of 
the  North  Washington  Power  &  Reduction  Co.,  at  Republic,  who  is 
familiar  with  the  ores,  having  studied  them  and  experimented  on  them  for 
years,  and  who  is  responsible  for  their  present  successful  treatment,  stated 
that  he  had  made  a  number  of  experiments  on  roasted  samples  and  in- 
variably found  that  while  a  slight  increase  of  gold  extraction  was  obtained 
on  subsequent  cyanidation,  invariably  the  extraction  of  silver  decreased 
and  almost  disappeared.  This  was  undoubtedly  one  of  the  reasons  why 
the  mill  built  by  Jackling  was  not  successful  as  the  silver  is  in  quantities 
too  great  to  be  ignored  and  constitutes  a  large  proportion  of  the  value. 

Another  point  which  has  been  detected  and  pointed  out  by  Mr.  New- 
ton is  that  the  gold  occurs  in  such  fine  particles  as  to  be  rarely  visible,  is 
scattered  through  an  extremely  close-grained  rock  so  that  the  finest 
grinding  is  necessary  to  liberate  it  and  expose  it  to  the  action  of  cyanide 
solutions. 

The  silver  probably  occurs  as  a  sulphide,  or  sub-sulphide,  and  is  soluble 
readily  enough  in  stronger  cyanide  solutions,  though  when  roasted  it  is 
converted  into  some  other  form,  probably  metallic  silver,  which  is  not 


METHODS  AT  REPUBLIC,  WASHINGTON  171 

readily  soluble  in  cyanide  solutions.  The  reasonable  treatment,  then, 
would  appear  to  be  sliming  of  the  total  ore  and  agitation  in  cyanide  solu- 
tions without  roasting  or  any  other  auxiliary  treatment.  And  this  has 
proved  to  be  the  case. 

The  Crushing  System. — The  mill  of  the  North  Washington  Power  & 
Reduction  Co.  was  commenced  in  1911  and  has  now  been  in  operation1 
something  more  than  seven  months.  It  handles  ore  from  the  Surprise  and 
Lone  Pine  mines,  a  rather  hard  run  of  ore.  Railroad  cars  deliver  the  ore 
to  the  mill  where  it  passes  through  a  system  of  crushing  and  sampling  as 
shown  by  the  accompanying  flow  sheet. 

From  the  first  bin,  the  ore  passes  over  a  grizzly  to  a  Blake  crusher  and 
then  over  another  grizzly  to  a  set  of  16X22-in.  rolls.  From  this  set  of 
rolls  it  is  passed  through  a  wire-screen  trommel,  the  apertures  of  which  are 
|Xj  in.  The  oversize  is  taken  to  a  set  of  14X30-in.  rolls,  after  passing 
which  it  joins  the  undersize  from  the  trommel  and  passes  through  the 
sampling  plant  to  the  mill  bin. 

From  the  mill  bin  the  ore,  now  reduced  to  such  size  that  the  largest 
pieces  are  about  T\  in.,  is  passed  to  a  Chilean  mill,  where  it  is  crushed 
to  pass  a  16-mesh  screen,  then  through  a  45-in.  Akins  classifier  and  the 
sand  to  a  tube  mill,  5X22  ft.,  where  it  is  slimed. 

There  is  some  difficulty  with  this  crushing  system,  the  crucial  point 
being  in  the  Chilean  mill.  The  capacity  of  the  plant  is  about  100  tons  per 
day  of  24  hr.,  and  yet  there  is  extreme  difficulty  in  getting  the  Chilean  mill 
to  crush  the  amount  of  coarse  material  contained  in  the  feed,  which  is  not 
large,  through  the  16-mesh  screen.  This  condition  should  not  exist,  for 
the  material  is  all  small,  as  has  been  mentioned,  and  the  screen  is  coarse. 
The  difficulty  probably  depends  on  two  factors,  one  of  which  is  the  fine- 
ness of  the  feed.  A  feed  of  this  fineness  is  usually  considered  too  small  to 
give  maximum  efficiency  with  a  Chilean  mill,  although  at  some  plants 
large  capacities  are  obtained  with  similar  material.  This  is  exemplified  at 
the  Portland  and  Independence  mills  at  Cripple  Creek.  Too  fine  feed  in  a 
Chilean  mill  usually  results  in  building  a  bed  on  the  die  ring,  the  rollers  rid- 
ing over  it  without  doing  much  grinding,  and  the  operators  at  the  North 
Washington  mill  state  that  this  is  what  occurs  in  this  case,  but  they  also 
state  that  increasing  the  size  of  the  material  decreases  the  capacity,  and 
conversely,  though  with  a  very  fine  feed  there  is  not  much  left  for  the 
mill  to  do  but  pulp  the  material  and  put  it  through  the  screen. 

The  probability  is  that  there  is  something  wrong  with  the  design  of  this 
Chilean  mill  and  that  it  is  not  adapted  to  ores  of  this  kind.  I  feel  con- 
vinced that  under  normal  conditions  there  would  be  no  difficulty  in  hand- 
ling a  sufficient  tonnage  of  this  material.  It  is  reduced  with  greatest  ease 
in  the  rolls. 

1  At  date  of  this  article. 


172 


DETAILS  OF  CYANIDE  PRACTICE 


Tube-mill  Practice. — At  present  there  is  one  tube  mill  in  operation, 
as  has  been  mentioned,  but  serious  consideration  is  being  given  to  the 
installation  of  a  second  one,  and  the  replacement  of  the  Chilean  mill  with 
a  short  tube  mill  of  large  diameter,  having  ribbed  lining  of  the  Komata  type. 
This  would  probably  be  an  advantageous  change  and  by  it  the  capacity  of 
the  mill  could  be  materially  increased.  A  short  mill  of  large  diameter 

f        1  Storage  Bin 

Shaking  Grizzly 
/Ox  20  "dlake  Crushe  r 
Shaking  Grizzly 
16x42"  Rolls 

4x6'  Trom  me  !-$  XjrApe  rtures 


Solution 
Tank 


14x30  RbHs 

18" Belt  Conveyor 

2~  Vezin  Samplers 
Mill  5in 

1-6' Trent  Chi  lean  Mill 

(16  Mesh  Ton  Cap  Screen) 

1-45" A  kins  Classifier 

1-5x22'  Tube  Hill  .ElOrolinmc. 

4-  Trent  Agitators  3$'x  20  '-Series 
Settling  E>ox 


r—  Tl  I-  11. 

^O  \        L_  _  >. 


-  11.5x16'  Oliver  Filter  • 


--~         i        Slime  Tails 

tJ  Excelsior  Cla  r  ifie  r 


Zinc  Dust 


Solution 


Pregnant  Solution  Tanks 
2~24-"  Per  rin  Presses 

t-Steele  Harvey  Furnace 


Bullion 
FLOW  SHEET  OF  THE  NORTH  WASHINGTON  POWER  &  REDUCTION  CO.'s  MILL. 

equipped  with  the  ribbed  lining  would  efficiently  break  up  the  coarser 
portion  of  this  mill-run  of  ore  and  leave  a  product  sufficiently  varied  to  be 
efficiently  slimed  in  the  succeeding  two  fine-grinding  mills. 

Closed  circuit  between  the  classifier  and  tube  mill  is  effected  and  the 
slime  which  leaves  the  classifier  is  taken  to  the  first  of  a  series  of  four 


METHODS  AT  REPUBLIC,  WASHINGTON  173 

agitation  tanks,  33X20  ft.,  which  are  equipped  with  the  Trent 
agitator. 

The  operators  claim  that  every  satisfaction  is  obtained  from  the  use 
of  these  agitators  even  in  the  large,  deep  tanks  installed.  They  perform 
their  work  with  a  minimum  of  trouble  and  give  good  results  in  extraction. 
The  aeration  of  the  pulp  is  said  to  be  good  and  extraction  proceeds 
satisfactorily. 

The  first  tank  is  used  as  a  thickener  as  well  as  agitator,  the  overflow 
going  to  the  precipitation  department.  Agitation  is  continuous  through 
the  series  of  tanks,  the  passage  through  them  being  accomplished  in 
about  36  to  40  hours. 

It  will  be  noted  that  the  agitation  capacity  of  these  tanks  is  sufficient 
for  a  good  deal  more  tonnage  than  can  be  delivered  by  the  crushing  and 
grinding  department.  The  slight  changes  necessary  for  increase  of  that 
part  of  the  mill  would  be  well  worth  making. 

Use  of  Cyanide  and  Other  Chemicals. — The  strength  of  the  cyanide 
solution  in  these  agitation  tanks  is  kept  at  about  5  Ib.  KCN  per  ton  of 
solution.  While  a  solution  containing  2  Ib.  per  ton  would  be  ample  for 
the  extraction  of  the  gold,  in  less  time  also,  the  strength  is  kept  up  to  5  Ib. 
in  order  to  make  the  best  possible  extraction  of  silver.  The  lime  is  added 
to  the  first  of  the  series  of  agitation  tanks  at  which  point  also  the  solution 
is  standardized  in  cyanide  content. 

Lead  acetate  is  used  intermittently.  It  has  been  found  that  after 
several  days'  use  of  the  acetate  it  can  be  discontinued  for  a  long  time,  the 
relation  being  approximately,  using  it  for  one  week  and  omitting  it  for 
three  weeks.  If  it  is  discontinued  for  much  more  than  this  time,  trouble 
develops  which  is  believed  to  be  due  to  soluble  sulphides,  the  condition 
being  immediately  remedied  by  the  addition  of  the  lead  salt. 

From  the  agitation  system  the  pulp  is  either  taken  directly  to  the 
Oliver  filter  or  is  first  passed  through  a  settling  box  for  thickening  it. 
The  procedure  is  usually  to  omit  the  settling  box  as  the  pulp  is  already 
in  good  condition  for  filtering. 

The  Oliver  filter  is  a  large  one,  11.5X16  ft.,  and  is  said  to  be  doing 
excellent  work. 

Precipitation  Details. — Zinc  dust  is  used  for  precipitation,  this  being 
the  most  acceptable  method  when  treating  ores  containing  copper,  as 
the  metal  is  recovered  immediately  from  the  solution  and  does  not  tend 
to  make  useless  a  large  amount  of  zinc,  as  is  the  case  when  shavings  are 
used.  The  Bosqui  system  of  agitating  zinc  dust  in  tanks  with  the  preg- 
nant solution  and  then  pumping  the  mixture  through  a  filter  press,  is 
followed. 

There  is  not  a  large  amount  of  copper  present,  but  what  little  there  is 
may  be  removed  in  this  way  from  solution.  A  statement  of  the  compara- 


174  DETAILS  OF  CYANIDE  PRACTICE 

tive  content  of  the  ores  milled  is  given  as  follows:  From  the  Surprise 
mine,  gold,  0.51  oz.;  silver,  5.80  oz.;  copper,  0.2%,  and  from  the  Lone 
Pine  Mine,  gold,  0.32  oz.;  silver,  2.20  oz.;  copper,  0.1%.  It  will  be 
seen  from  this  statement  that  the  copper  is  not  in  sufficient  quantity  to 
cause  any  great  difficulty. 

The  average  grade  of  ore  treated  at  the  mill  for  six  months  has  been 
$9.55,  and  the  extraction  of  gold  during  this  time  was  92.8%  and  of  silver 
90.5%.  The  consumption  of  cyanide  is  1.3  Ib.  per  ton  of  ore  treated,  of 
lime,  4.5  Ib.,  and  of  Zn,  0.6  pound 

The  mill  consumes  2  hp.  for  each  ton  of  ore  milled  and  employs  12 
men  in  its  actual  operation  in  addition  to  six  men  at  the  power  plant. 
Power  is  electric,  generated  by  a  not  very  efficient  steam  plant  at  the  mill, 
and  is  expensive.  It  is  expected  that  hydro-electric  power  will  be  avail- 
able within  the  near  future  when,  based  on  the  expenses  at  the  present 
time,  the  total  cost  of  treating  the  ore  will  not  exceed  $2  per  ton. 

Extraction  percentages  are  based  on  the  samples  taken  at  the  head  of 
the  mill  by  the  sampling  plant  and  calculated  against  the  tailing  samples 
taken  below  the  filter.  It  is  said  that  the  bullion  return  checks  closely 
with  these  calculated  results. 

The  precipitate  is  not  acid  treated,  but  is  melted  with  flux  in  a  Steele 
Harvey  furnace.  The  refinery  department  is  situated  at  the  head  of  the 
mill  and  in  a  separate  building. 

A  New  Crushing  Machine. — The  mill  of  the  SanPoil  Consolidated  Co., 
the  only  other  plant  operating  at  the  present  time  in  the  Republic  district, 
is  situated  in  a  canon  some  little  distance  north  of  the  town.  The  mill  has 
been  recently  completed  and  is  operating  on  ores  of  much  the  same 
character  as  those  referred  to  in  connection  with  the  North  Washington 
mill,  hard,  close-grained  quartz. 

Railroad  cars  deliver  the  ore  into  the  bin  at  the  head  of  the  plant  and 
from  this  bin  an  18-in.  belt  conveyor  delivers  it  to  a  Williams  hammer 
trommel  mill.  This  machine  is  unusual  in  milling  plants.  It  is  constructed 
much  like  the  Quenner  dry  placer  machine  which  was  devised  at  the 
Altar  field  in  Sonora,  Mexico.  It  consists  of  a  trommel  built  of  heavy  bars 
spaced  yV  in.  apart,  revolving  at  100  r.p.m.,  and  a  central  shaft  running 
through  it  revolving  at  600  r.p.m.  Upon  the  central  shaft  are  32  hammers 
weighing  16  Ib.  each  connected  with  the  shaft  by  heavy  chains.  The 
hammers  swing  so  that  there  is  no  danger  of  their  touching  the  trommel. 
Ore  is  introduced  at  one  end  of  the  machine  and  the  hammers,  flying  out 
by  centrifugal  force,  strike  the  pieces  of  rock  and  shatter  them.  Several 
kinds  of  metal  have  been  used  in  making  these  hammers,  which  receive  the 
major  part  of  the  wear,  and  at  present  the  most  available  material  has 
proved  to  be  mild  steel,  sawn  from  the  bar,  and  drilled  to  receive  the  chain 
connections.  Wear  of  the  hammers  has  diminished  from  an  original  two 


METHODS  AT  REPUBLIC,  WASHINGTON 


175 


or  more  pound  per  ton  of  rock  crushed  to  a  point  where  it  is  not  much  more 
than  0.5  Ib.  per  ton. 

The  machine  certainly  does  an  immense  amount  of  work,  reducing 
mine  run  to  a  granular  product  at  one  operation.  The  product  is  supposed 
to  come  out  through  the  trommel  bars  at  y\  m->  the  distance  between 
the  bars,  but  there  is  a  good  deal  of  it  which  is  considerably  larger,  pre- 
sumably due  to  wearing  of  the  bars,  thus  enlarging  the  openings.  In 

Ore  Bin  -600  Tons 
!8"  Belt  Conveyor 


Sol.  Storage 


Solution 


Sump 


l-Williams  Hammer 
Trommel  Mill 

l-Elevator-Belt8cBucket-20Ton-hrCap 

Storage  Bin 

14-24  Rolls.  Mils-Chalmers  "C " 

t- 5x22" Gates  Tube  Mi!l,smooth 


g       /-Sana"  Wheel 
•> 

I- Duplex.  Dorr  Classifier 

1-25  x  10 'Dorr  Thickener 

9-l2x  18' Agitators,  tirLiftSeries 
^      Cone  Bottom  Thickener 
^     I- 16' Oliver  Filter 


Slime  Tailing 


C/an'-f/er 


Zinc  Boxes 


Melting  Furnace 
Bullion 

FLOW  SHEET  OF  THE  SAN  FOIL  CONSOLIDATED  MILL. 

operation  the  machine  makes  an  enormous  amount  of  dust  as  the  dry  ore 
is  broken  up.  When  damp  ore  is  used  the  quantity  of  dust  is  not  so  great 
and  the  general  product  is  finer,  but  its  capacity  is  much  reduced.  Sizing 
tests  on  the  product  show  as  high  as  40%  of  the  product  passing  a  20- 
mesh  screen,  of  which  65%  is  finer  than  10-mesh  and  6%  through  200- 
mesh  screen.  About  70  tons  per  day  are  being  milled  through  the 
machine.  Its  capacity  is  10  tons  per  hour. 

It  is  acknowledged  that  this  machine  is  still  in  the  experimental  stage, 
although  the  operators  believe  it  will  continue  to  perform  its  work  as  at 


176  DETAILS  OF  CYANIDE  PRACTICE 

present.  The  machine  now  installed  is  to  be  exchanged  for  another 
similar  one  built  much  heavier,  but  designed  to  do  the  same  work  in  the 
same  way.  It  will,  however,  require  incontestable  proof  to  show  that  a 
machine  of  this  kind  can  be  built  strong  enough  to  stand  the  enormous 
strain  of  reducing  mine-run  ore  to  J  in.  or  even  J  in.  at  one  operation  by 
such  strenuous  means  and  be  economical. 

Sampling  the  fine  ore  is  done  automatically,  and  after  passing  the  rolls, 
as  shown  in  the  accompanying  flow  sheet,  it  is  sent  to  a  5X22-ft.  tube  mill 
which  is  in  closed  circuit  with  a  duplex  Dorr  classifier. 

Air  Agitation  Methods. — -The  slime  from  the  classifier  is  thickened  in  a 
Dorr  thickener  25  X 10  ft.,  and  the  thickened  pulp  taken  to  a  series  of  nine 
agitator  tanks,  each  12  X 18  ft.,  with  cone  bottoms  and  using  air  agitation. 
In  addition  to  the  central  air  lift  in  these  agitators,  which  are  cut  off  below 
the  surface  as  is  done  in  the  tanks  at  Grass  Valley,  to  be  mentioned  in  the 
following  paper,  there  are  three  additional  lifts  in  each  tank,  designed  to 
avoid  settling  of  slime  on  the  cone  bottoms  which  are  too  flat  to  do  this 
automatically.  Treatment  through  the  tanks  is  continuous  and  solution 
from  5  to  7  Ib.  KCN  per  ton  is  used.  At  this  mill  the  stronger  solutions 
are  used  in  the  tube  mill,  the  weaker  ones  being  made  up  at  the  agitators. 
The  time  of  agitation  is  50  to  70  hr.  in  the  tanks. 

From  the  final  agitator  of  the  series  the  pulp  is  taken  to  a  cone-bottom 
thickener  and  thence  to  an  Oliver  filter,  16-ft.  drum,  where  filtration  is 
effected.  The  pregnant  solution  is  passed  through  zinc  boxes  using  shav- 
ings and  the  barren  solution  issuing  is  passed  back  to  the  final  agitator  of 
the  series  of  nine,  thus  diluting  the  pulp  and  giving  it  a  wash  before  going 
to  the  thickening  box  and  filter.  Solution  overflowing  from  the  thickener 
is  pumped  back  to  storage  to  be  used  over  again. 

The  ore  averages  $8  per  ton  in  value  and  92  to  95%  is  extracted. 
Consumption  of  KCN  is  1 . 5  Ib.  per  ton  and  CaO  2.3  Ib.  per  ton. 

There  is  nothing  particularly  novel  about  this  plant  with  the  exception 
of  the  Williams  mill  above  mentioned,  but  it  is  another  link  in  the  chain  of 
recent  events  which  have  shown  clearly  that  the  ores  of  jthe  Republic  do 
not  deserve  the  reputation  they  have  received  for  being  difficult  to  treat 
and  that  when  use  is  made  of  the  means  which  have  been  developed  within 
the  last  few  years  for  the  better  working  of  the  cyanide  process,  together 
with  the  essential  and  vital  point  of  the  metallurgy  as  applied  to  this 
particular  rock,  extremely  fine  grinding,  there  is  no  difficulty  in  benefi dat- 
ing it  cheaply  and  efficiently. 

Neither  of  the  mills  at  present  in  operation  is  perfect  nor  is  to  be  taken 
as  a  model  of  its  kind,  but  considering  the  difficulties  in  the  way  of  getting 
sufficient  capital  to  treat  ores  which  had  already  been  tagged  as  hopeless, 
they  are  good,  and  too  much  credit  cannot  be  given  to  the  men  who  have 
made  a  metallurgical  success  where  inevitable  failure  was  freely  foretold. 


CHAPTER  XVI 
THE  MILLS  OF  GRASS  VALLEY,  CALIFORNIA 

The  North  Star  Mines  Co.,  situated  near  the  town  of  Grass  Valley  in 
Nevada  County,  Calif.,  operates  two  mills,  one  built  at  the  shaft  of  the 
mine,  known  as  the  North  Star  mill,  and  one  situated  in  a  canon  near  the 
town,  known  as  the  Central  mill.  The  Central  is  the  newer  mill,  built 
much  later  than  the  old  North  Star  mill.  They  are  both  40-stamp  instal- 
lations and  treat  about  the  same  quantity  of  ore  in  exactly  the  same  way, 
the  only  difference  being  that  the  older  mill  does  not  attempt  to  treat  the 
concentrate  made  in  it,  but  sends  it  to  the  Central  mill  for  reduction. 
Owing  to  these  facts  the  Central  mill  will  be  discussed  in  his  paper,  its 
system  typifying  the  practice  established  by  the  company  and  illustrating 
the  method  of  concentrate  treatment. 

Occurrence  of  Gold. — The  ore  is  a  free-milling  quartz  in  which  quan- 
tities of  sulphides  occur,  and  it  is  principally  in  these  sulphides  that  its 
value  lies.  The  principal  valuable  metal  is  gold;  silver  exists  in  small 
proportion,  but  not  sufficient  to  justify  any  special  metallurgical  efforts  to 
recover  it.  The  gold  is  principally  in  mechanical  association  with  the 
sulphides,  only  a  small  proportion  being  in  chemical  combination.  In 
crushing,  the  ore  may  be  said  to  be  of  medium  hardness.  It  consists 
principally  of  quartz. 

Each  of  the  mills  of  the  company  contains  40  stamps,  and  the  com- 
bined capacity  of  the  two  is  about  9000  tons  per  month,  or  about  300 
tons  per  day.  The  average  ore  treated  in  the  mill  is  valued  at  about  $10 
to  $12  per  ton. 

Stamp -crushing  Practice. — The  ore  is  brought  to  the  Central  mill  in 
small  cars  drawn  by  a  light  electric  locomotive.  It  is  dumped  into  a 
general  bin  from  which  it  passes  over  a  grizzly,  the  coarse  rock  being 
reduced  in  size  in  crushers  of  the  Blake  type  and  the  whole  finally  received 
in  the  battery  bins. 

The  construction  of  these  bins  is  noteworthy  in  that  they  are  entirely 
of  masonry,  forming  a  massive  and  solid  foundation  for  the  crushers. 
They  are  built  directly  on  the  hillside  and  are  a  fine  example  of  permanent 
construction. 

From  the  battery  bins  the  ore  is  drawn  through  disk  feeders  into  the 
mortars.  There  are  40  stamps  of  1000  Ib.  each  dropping  110  times  per 
min.  through  7  in.  Inside  amalgamation  is  practised,  using  chuck  plates, 

177 


178 


DETAILS  OF  CYANIDE  PRACTICE 


and^there  is  also  an  outside  lip  plate.     The  regular  apron  plates  follow  as 
is  usual. 

The  screen  in  use  is  somewhat  unusual,  consisting  of  a  light  sheet- 
•  steel  plate  punched  with  small  round  holes.  These  holes  are  grouped  in 
small  squares  of  about  1  in.  on  a  side,  and  the  squares  are  separated  by 
ribs  of  the  solid  metal,  about  as  shown  in  the  illustration.  The  size  of 


CENTRAL  MILL. 


CENTRAL  CYANIDE  PLANT. 
NORTH  STAR  MINES,  GRASS  VALLEY,  CALIF. 

the  holes  is  such  that  they  approximate  the  opening  generally  found  in  a 
30-mesh  wire  screen.  The  separating  ribs  add  strength  to  the  screen. 
The  whole  length  of  the  mortar,  with  a  width  of  about  one  foot,  is  occupied 
by  the  screen  which  has  no  strengthening  strips  aside  from  the  ordinary 
wooden  frame  used  to  retain  it. 

The  battery  framing  is  entirely  of  steel.     The  driving  arrangement  is 
a  little  out  of  the  ordinary  as  the  motor  is  situated  above  the  batteries  and 


THE  MILLS  OF  GRASS  VALLEY,  CALIFORNIA 


179 


STAMP  BATTERY  AND  PLATES,   CENTRAL  MILL  OF  NORTH  STAR  MINES. 


o  o 
o  o 
o  o 
o  o 


O  O  O  O" 

o  o  o  o 

0000 


o  o 
o  o 


L 


000 

-  /"  - 


H 


o  o  o  o  o  o 

000000 
000000 

o  o 
o  o 
o  o 


o  o  o  o 
o'  o  o  o 

0000 


o  o  o  o  o  o 

000000 

o  o  o  o  o  o 

000000 
000000 

o  o  o  o  o  o 


'•  Wooden  Frame 
K~  O  O  O  O  O  O 

;  o  o  o  o  o  o 

'   O  O  O  O  O  O 

-.  o  o  o  o  o  o 
i  c  o  o  o  o  o 
o  o  o  o  o 


o  o  o  o  o  o 

o  o  o  o  o  o 

o  o  o  o  o  o 

o  o  o  o  o  o 

000000 
000000 

000000 

o  o  o  o  o  o 

000000 
000000 

o  o  o  o  o  o 

000000 


o  o  o  o  o  o 

o  o  o  o  o  o 

o  o  o  o  o  o 

o  o  o  o  o  o 

000000 
000000 


000000 
000000 


o  o  o 

000 


o  o 
o  o 


o  o  o  o  o  o 

000000 


o  o  o  o  o  o 

000000 

o  o  o  o  o  o, 

0000 

o  o 


SCREEN  USED  AT  NORTH  ST^AR  MILL. 


180  DETAILS  OF  CYANIDE  PRACTICE 

the  driving  belts  lead  down,  with  one  reduction,  to  the  camshaft.  The 
accompanying  illustration  shows  this  arrangement.  By  this  means  the 
battery  motor  is  given  a  firm  foundation  on  the  masonry  of  the  bins  and 
all  belts  and  pulleys  are  easily  reached  when  desired. 

From  the  plates  the  pulp  is  led  to  a  series  of  concentrators  which  are 
round,  of  the  buddle  type,  but  have  a  reciprocating  motion,  revolving 
back  and  forth  about  a  central  point,  through  a  small  arc.  The  concen- 
trate is  removed  at  a  point  on  the  edge  of  the  circle.  An  arrangement  is 
made  for  reconcentrating  the  middling  of  a  number  of  these  tables  on 
others.  The  machine  is  known  as  the  Dodd  buddle. 

From  the  concentrators  the  pulp  flows  to  the  cyanide  plant,  which  is 
separate  from  the  mill  and  situated  at  some  distance  from  it.  All  crush- 
ing and  concentration  is  done  in  water  and  the  pulp  flows  by  gravity  to 
the  cyanide-treatment  plant. 

Cone  System  of  Classification. — All  the  pulp  is  received  at  the  cyanide 
plant  in  a  roughing  cone,  7X6  ft.  The  overflow  from  this  cone  goes 
immediately  to  the  cyanide  department  as  slime,  the  object  being  to  take 
out  the  majority  of  the  slime,  and  slime  only,  leaving  the  cone  underflow 
to  be  further  classified  in  a  separate  system.  This  system  consists  of 
three  cones,  each  5X5  ft.,  using  an  upward  current  of  water  to  assist  in 
the  classification.  One  of  these  cones  has  straight  sides,  but  of  approxi- 
mately the  same  size  as  the  other  two,  and  its  operation  is  the  same. 
Clean  separation  is  accomplished  in  these  cones  and  their  overflow  joins 
the  stream  of  slime,  from  the  roughing  cone  and  forms  the  slime  portion  of 
the  pulp. 

Separate  Sand  and  Slime  Treatment. — The  underflow  of  the  cones  is 
taken  directly  to  the  leaching  tanks  where  the  first  cyanide  solution  is 
put  in  contact  with  it. 

The  leaching  tanks  are  six  in  number,  each  22  ft.  5  in.  by  87  in.  and 
are  served  by  distributors  of  the  Butters  type.  The  cyanide  treatment 
occupies  about  seven  days  in  total.  At  first  a  wash  of  lime  water  is 
applied  to  neutralize  any  acidity  in  the  ore  and  prepare  it  for  the  cyanide 
solution  which  follows.  After  this  has  been  drained  away  a  bath  of  30 
tons  of  pregnant  solution  from  the  filters  is  applied  which  has  strength  of 
about  0.035%  KCN.  This  is  immediately  drained  and  is  followed  by  a 
bath  of  barren  solution  containing  0.1%  KCN.  The  tank  is  filled  with 
this  solution,  which  is  allowed  to  remain  in  contact  with  the  charge  for 
24  hr.  with  the  tank  outlets  closed  meanwhile.  After  this  soaking  period, 
the  solution  is  allowed  to  drain  into  a  special  solution  tank  until  it  shows 
a  strength  of  0.06%  KCN,  when  it  is  turned  into  the  strong  sump,  where 
it  finally  reaches  a  strength  of  about  0.09%  KCN.  Other  strong  washes 
are  followed  by  washes  of  sblution  from  the  filters  and  finishes  with  the 
barren  solutions  which  are  to  be  wasted,  after  being  precipitated,  and 


THE  MILLS  OF  GRASS  VALLEY,  CALIFORNIA  181 

finally  a  water  wash  is  applied.     The  sand,  after  treatment,  is  sluiced  out 
of  the  tanks.     There  is  no  scarcity  of  water. 

The  slime  is  all  collected  in  a  series  of  cone-bottomed  settling  tanks, 
assisted  by  one  Dorr  thickener.  There  are  six  of  these  settling  tanks, 
three  of  which  are  14  X 15  ft.  and  three  13.3  X9  ft.  The  Dorr  thickener  is 
19.5X13.5  ft.,  and  is  arranged  to  make  one  revolution  in  3  min.  50  sec. 
The  slime  is  thus  dewatered,  leaving  a  pulp  having  specific  gravity  of 
about  1.3  and  a  dilution  of  2  : 1,  approximately. 

In  a  system  of  this  sort,  where  an  appreciable  amount  of  water  is  taken 
into  the  cyaniding  system,  it  is  evident  that  it  will  have  to  be  discarded 
at  some  point.  This  introduces  a  condition  somewhat  difficult  to  handle 
and  it  is  only  where  weak  solutions  are  used  that  it  is  practicable.  At  the 
Central  mill  weak  solutions  are  used  in  slime  treatment  and  for  leaching 
the  sand.  With  sand  treatment  by  leaching  it  is  possible  to  reduce  the 
incoming  moisture  to  about  the  same  quantity  discharged  with  the  tail- 
ings, so  that  in  this  department  there  is  little  or  no  accumulation  of  sur- 
plus solutions.  This  can  be  accomplished  with  slime  also,  but  is  an  expen- 
sive process,  in  fact  it  was  tried  at  the  Central  mill  at  one  time  but  it 
was  soon  found  to  be  more  economical  to  waste  a  selected,  precipitated 
solution,  with  its  cyanide  content  reduced  to  the  lowest  possible  point, 
than  to  undertake  the  expense  of  more  complete  dewatering. 

Slime  Treatment  System. — The  partially  dewatered  slime  is  recovered 
in  a  stock  tank,  from  which  it  is  taken  to  the  agitation  department  and 
the  necessary  cyanide  added  to  bring  up  the  strength  to  the  treatment 
standard  of  0.035%  KCN.  The  slime,  with  its  added  cyanide  and  solu- 
tion, goes  then  to  the  treatment  tanks. 

The  tanks  used  for  agitation  are  8  ft.  diameter  and  18  ft.  deep  and 
have  cone  bottoms  of  60°  inclination.  The  agitation  is  by  means  of  a 
central  lift  pipe.  It  will  be  noted  that  these  tanks  are  practically  Pachuca 
tanks,  except  that  the  lift  pipe  does  not  reach  the  surface  of  the  pulp,  a 
feature  which  has  been  adopted  with  some  advantage  in  a  few  Pachuca 
installations.  Air  is  used  for  operating  the  lifts.  It  is  of  interest  to  note 
that  these  tanks  are  said  to  have  been  installed  and  in  use  before  the 
Pachuca  type,  as  such,  came  into  existence. 

The  agitation  of  the  slime  pulp  is  continuous  through  the  three 
agitators,  the  time  of  passage  being  calculated  at  approximately  14  hours. 

Continuous  Filter  System. — The  pulp  from  the  agitators  is  collected 
in  two  settlers,  used  alternately,  each  8X13  ft.,  equipped  with  cone  bot- 
toms. Settling  and  decantation  proceed  simultaneously,  the  thickened 
pulp  meanwhile  being  drawn  off  for  nitration.  In  about  eight  hours,  one 
settler  is  filtered  so  that  the  overflow  is  not  clear,  when  the  pulp  is  trans- 
ferred to  the  other  settler,  the  first  one  being  allowed  to  settle,  the  solution 
decanted,  and  the  thickened  underflow  drawn  off  as  it  settles. 


182 


DETAILS  OF  CYANIDE  PRACTICE 


Ore  from  Mine 


Water  Waste  or 
^  ^  Auxiliary  Use 
^~s    E; 


Rough- ore  Bin 

Grizzly 

2-9x15'  Blake  -  type  Crushers 
8-  Disc  Feeders 

40- 1000  Ib.  Stamps-  7" Drop- 110 
Drops  -30- -flesh  Screen,  Chuck  and 
Lip  Amalgamation  Plates 
8 -Outside  Amalgamation  Plates 

12 -D odd  Buddies, Pipe  Line  to 
Cyanide  Plant 

1-7x6'  Roughing  Cone 
3-5x5  Hydraulic  Classifying 

6-22-Jx  7-3" Leaching  Tanks 
Butters  Distributors 


J  6-  Settling  C 
\l-Dorr  Thic 


ones 
Thickener 


SI  i  me  Stock  Tank  9-6  "x  JO' 

3-%'xlQ'A  ir  Agitation  Tanks,  60° 
Cone  bottoms,continuous  rlow 

2-8x12  'Settlers,  alternate  Use 

2-102x7'  0  fiver  Filters 
,  Slime  Tails 


Barren  ,    .-•>* 
Solution  L 


\     i  Tube  Zinc  Grinder  and 
Feeder 

Sump  Tanks 


2- 36 '-6  Frame  Merrill  Precipi  ta  te 
Presses 

"Precipitate 


Oil  -  fired  Sta  tionory  Furnaces 


Bullion 

FLOW  SHEET  OF  CENTRAL  MILL. 


THE  MILLS  OF  GRASS  VALLEY/CALIFORNIA  183 

The  thickened  pulp  is  taken  to  the  filters,  this  installation  consisting  of 
two  10 J  X7-ft.  Oliver  continuous  revolving  machines.  It  was  at  this  mill 
that  the  Oliver  filter  was  conceived  and  developed  and  the  two  machines 
now  in  use  are  the  first  of  their  type  to  be  built. 

The  tailing  is  sluiced  away  from  the  filter  and  run  to  waste.  About 
60  tons  of  slime-  daily  are  treated  on  the  two  filters. 

The  Merrill  system  of  precipitation  is  used.  Zinc  dust  added  to  the 
pregnant  solution  is  pumped  through  two  Merrill  presses,  each  of  which 
contains  eight  frames,  the  frames  being  the  36-in.  triangular  type.  These 
presses  originally  had  the  solution  discharge  at  the  bottom  of  the  frame, 
but  pipes  have  been  fitted  to  the  discharges  and  carried  above  the  level  of 
the  frames  so  that  the  press  always  remains  full  even  in  case  of  cessation  of 
operations  of  the  plant.  The  frames  thus  kept  full  of  solution  prevents 
oxidation  of  the  zinc  consequent  upon  an  empty  frame  and  resulting  con- 
tact of  air  with  the  zinc.  All  solutions  to  be  precipitated  are  first  used  as 
washes  on  the  sand-leaching  tanks,  enriching  the  solutions  and  effectually 
clarifying  them.  The  precipitate  is  melted  in  stationary,  oil-fired  fur- 
naces without  any  preliminary  treatment  whatever.  A  small  tube  mill  is 
used  for  emulsifying  the  zinc  dust  with  solution  before  its  addition  to  the 
solution  to  be  precipitated,  and  a  mechanical  feeder  introduces  the  zinc 
into  the  tube  mill. 

Concentrate  Treatment. — The  concentrate  treatment  is  unusual  and 
worthy  of  particular  note.  The  two  mills  of  the  company  make  six  to 
seven  tons  of  concentrate  daily,  all  of  which  is  treated  at  the  Central  mill. 
It  is  brought  to  the  mill  in  cars  and  is  dumped  into  a  concentrate  bin, 
being  meanwhile  mixed  with  lime  at  the  rate  of  about  nine  pounds  per 
ton.  From  this  bin  the  concentrate  is  mechanically  fed  into  a  tube  mill  of 
the  Abbe  type.  The  mill  is  4.5  ft.  in  diameter  and  20  ft.  long  and  revolves 
at  19  r.p.m.  The  concentrate  is  reground  in  this  mill,  and  issuing,  it  is 
classified  in  a  cone  classifier:  The  fine  overflow  all  goes  over  an  amalga- 
mating plate  and  the  underflow  is  returned  to  the  tube  mill,  forming  a 
closed  circuit  from  which  there  is  no  exit  save  through  the  overflow  of  the 
reclassifying  cone.  All  of  the  reground  concentrate  pulp,  after  having 
passed  this  regrinding  and  amalgamating  system,  is  taken  directly  to  the 
large  roughing  cone,  before  mentioned,  at  the  head  of  the  cyanide  plant, 
where  it  enters  treatment  along  with  the  bulk  of  the  ore. 

Thus  the  concentrate,  after  having  been  removed  from  the  pulp,  is 
ground,  amalgamated,  and  returned  to  it  and  undergoes  exactly  the  same 
treatment  as  the  siliceous  part  of  the  ore.  This  treatment  is  said  to  be 
altogether  satisfactory,  the  regrinding  being  sufficient  to  liberate  most 
of  the  contained  gold,  a  part  of  which  is  recovered  on  the  plate  and  the 
remainder  in  the  cyanide  treatment.  The  portion  which  goes  to  the 
cyanide  plant  is  amenable  to  the  short-time  and  weak  solutions  used  there. 


184 


DETAILS  OF  CYANIDE  PRACTICE 


The  explanation,  of  course,  is  that  the  gold  is  only  mechanically  com- 
bined with  the  sulphides  and  is  liberated  by  the  additional  grinding. 
In  the  case  of  chemical  combinations  other  means  would  be  necessary  for 
its  satisfactory  extraction. 

This  is  a  good  illustration  of  the  utility  of  experimental  work,  for  it  has 
here  fitted  an  inexpensive  process  to  a  material  which  needs  nothing  else. 
A  separate  treatment,  such  as  is  usually  considered  necessary  with  sul- 
phides, might  have  been  considered  necessary  in  this  case  and  an  expensive 
plant  and  treatment  system  might  have  been  installed  had  not  thorough 
experiment  shown  the  exact  requirements  of  the  material. 

The  concentrate  varies  in  value  from  $30  to  $80  per  ton  and  the  ex- 
traction is  about  as  complete  as  with  the  siliceous  portion  of  the  ore.  As 
has  already  been  said,  the  mill  head  averages  $10  to  $12  per  ton,  and  the 
head  of  the  cyanide  plant  averages  about  $2  per  ton,  before  the  concen- 
trate is  added.  The  total  extraction  obtained  from  mill  and  cyanide 
plant  approaches  98%.  In  the  cyanide  plant  alone,  about  94%  of  the 
gold  in  slime  is  recovered  and  85  to  90%  of  that  in  the  sand.  The  tailing 
contains  on  the  average  about  $0.20  in  gold  value. 

Consumption  of  Material  and  Cost. — The  consumption  of  cyanide  is 
0.61  Ib.  per  ton  milled;  of  zinc,  0.216  Ib.  per  ton;  of  lime,  4.21  Ib.  per  ton, 
and  of  pebbles,  4.5  Ib.  per  ton  of  sulphides  ground. 

The  cost  of  treatment  is  given  in  the  accompanying  table. 


Milling 

Concentration 

Cyaniding 

Labor 

$0  176 

$0  087 

$0  166 

Material  
Power 

0.075 
0  129 

0.017 
0  027 

0.186 
0  116 

Total 

$0  380 

$0  131 

$0  468 

Milling  

$0.380 

Concentration  

0  131 

Cyanidation  
Total.  . 

0.468 
10.979 

The  Empire  Mill. — The  Empire  mill  is  also  near  the  town  of  Grass 
Valley.  The  mine  and  mill  are  close  together,  although  as  in  the  case 
of  the  Central,  the  milling  and  cyanide  departments  are  in  separate 
buildings. 

The  ore  at  the  Empire  mill  is  like  that  at  the  North  Star,  a  free-milling 
quartz  containing  sulphides.  The  mill  is  similar,  and  in  fact,  the  treat- 
ment is  almost  identical,  so  that  it  will  not  be  necessary  to  make  a  detailed 
study  of  that  practice,  but  attention  will  only  be  called  to  the  few  points 
of  difference. 

The  Empire  mill  contains  40  stamps  of  1050  Ib.  each  and  amalgama- 
tion is  practised  both  inside  and  outside  the  mortars.  The  quantity  of 


THE  MILLS  OF  GRASS  VALLEY,  CALIFORNIA 


185 


ore  milled  is  from  140  to  150  tons  per  day,  about  the  same  as  the  North 
Star  mills.     Concentration  is  practised  as  is  the  case  at  the  other  mills. 

The  mill  pulp  flows  through  a  pipe  line  to  the  cyanide  plant,  where 
it  is  received  in  two  roughing  cones,  the  slime  overflowing  and  going  di- 
rectly to  settlers  and  the  underflow  passing  for  further  classification  to 
four  hydraulic  classifying  cones,  which  make  the  final  separation  into 
sand  and  slime. 


EMPIRE  MILL. 


EMPIRE  CYANIDE  PLANT. 
GRASS  VALLEY,  CALIF. 

The  sand  is  taken  directly  to  the  leaching  tanks  and  is  delivered  into 
them  by  means  of  Butters  distributors.  A  feature  of  the  practice  both  at 
the  Empire  and  Central  mills  is  that  the  Butters  distributors  are  used 
both  in  filling  the  tanks  and  also  in  sluicing  out  the  residue.  For  sluicing 
out  the  sand,  water  is  admitted  through  the  distributor,  the  bottom  gate 

13 


186  DETAILS  OF  CYANIDE  PRACTICE 

opened,  and  sluicing  proceeds  automatically.  The  filter  bottom  of  the 
tank  is  made  slightly  inclined,  in  order  to  facilitate  the  discharge.  The 
sand  tanks  are  14  ft.  by  9  ft.  4  in.,  and  contain  approximately  155  tons. 
The  treatment  occupies  six  days  and  the  strong  solution  is  0.10%  KCN, 
as  in  the  case  of  the  Central  mill. 

Settling  Area. — The  slime  from  the  classifiers  is  taken  to  the  settlers. 
There  are  four  of  these,  each  24  ft.  in  diameter  and  22  ft.  deep,  with  cone 
bottoms.  The  pulp  is  distributed  among  them  as  conditions  dictate. 
This  settling  capacity  seems  extensive  for  the  quantity  of  slime  handled, 
about  65  to  70  tons  daily,  but  it  is  stated  that  the  settling,  even  with  this 
great  capacity,  is  accomplished  only  with  great  difficulty.  A  large  quan- 
tity of  lime  is  added  to  assist  settlement,  most  of  this  lime  being  lost  in  the 
overflow  water,  which  is  either  run  to  waste  or  used  in  auxiliary  opera- 
tions about  the  plant,  there  being  none  returned  for  further  use  in  milling. 

It  is  an  object  to  draw  the  slime  from  these  settlers  with  as  little  mois- 
ture as  possible,  the  dilution  usually  being  about  1:1.  As  the  thickened 
pulp  reaches  the  agitation  department  it  is  diluted  to  about  2:1  with 
fresh  solution  and  is  made  up  to  0.04%  cyanide  strength. 

The  agitation  is  continuous  through  four  tanks  of  the  Pachuca  type, 
each  10X18  ft.  The  cyanide  strength  is  made  up  in  the  first  and  last 
tanks  of  the  series  and  a  low  protective  alkalinity  maintained.  There  is 
no  thickening  or  washing  of  the  pulp  after  agitation  except  that  given  in 
the  filters,  to  which  the  pulp  is  taken  directly  from  the  agitation  tanks. 
The  filter  installation  consists  of  two  Oliver  machines  of  the  8-ft.  size. 

As  at  the  Central  mill,  all  solutions  to  be  precipitated  are  first 
used  as  washes  on  the  sand  tanks  in  order  to  clarify  them.  The  Merrill 
system  of  zinc-dust  precipitation  is  used,  and  there  are  two  triangular 
frame  presses,  36-in.  frames,  having  nine  and  ten  frames  respectively. 
These  presses  have  pipe  connections  for  the  solution  outlets  which  deliver 
above  the  level  of  the  top  of  the  frames.  The  precipitate  is  acid  treated 
and  melted. 

The  concentrate  treated,  about  four  tons  daily  (part  of  which  is  pur- 
chased), is  reground  in  a  4  X  8-ft.  Allis-Chalmers  tube  mill,  run  over  plates 
and  classified  at  the  cyanide  plant  in  a  separate  cone,  the  overflow  of 
which  joins  the  stream  of  slime  to  the  settlers  and  the  underflow,  the 
product  going  to  the  sand-leaching  tanks.  This  separate  classification  is 
the  only  detail  in  which  the  concentrate  treatment  differs  from  that  in 
use  at  the  Central  mill. 

The  general  arrangement  at  the  Empire  mill  is  somewhat  less  com- 
plicated, as  it  is  understood  that  the  mill  stands  exactly  as  designed  and 
the  necessity  for  inconvenient  remodeling  has  been  avoided. 

The  Grass  Valley  properties  present  mining  in  its  most  attractive  form. 
Situated  in  a  fertile  agricultural  district,  free  from  climatic  extremes  and 


THE  MILLS  OF  GRASS  VALLEY,  CALIFORNIA  187 

surrounded  by  luxuriant  vegetation,  the  aspect  differs  widely  from  the 
usual  mining  camp.  The  district  is  ola,  the  first  discovery  was  in  1850, 
and  proven,  so  that  makeshifts  and  cheap  construction  have  long  since 
given  place  to  substantial  installations,  an  artistic  eye  having  evidently 
been  concerned  in  the  general  plans.  This  is  especially  true  at  the  Empire 
where  stone  buildings  and  pleasant  foliage  give  an  appearance  more  like 
that  generally  presented  by  well  designed  and  permanent  habitations  than 
mining  camps.  Those  concerned  in  the  operation  of  these  mines  are 
fortunate  indeed  in  their  habitation. 


CHAPTER  XVII 
THE  BLACK  OAK  PLANT,  CALIFORNIA 

Near  the  town  of  Soulsbyville,  in  Tuolumne  County,  Calif.,  is  situated 
the  plant  of  the  Black  Oak  Development  Co.,  said  to  be  the  only  mill  at 
the  present  in  the  State  of  California  which  employs  the  total-sliming, 
continuous-agitation  process  of  treatment.  The  property  is  in  an  agree- 
able district  where  climatic  conditions  are  good  and  the  natural  surround- 
ings pleasant. 

Ore  Character. — The  plant  is  near  the  town  of  Tuolumne,  the  terminus 
of  the  branch  line  railroad  by  means  of  which  the  district  is  reached,  from 
Stockton,  via  Oakdale. 

Gold  and  silver  both  occur  in  the  ore  of  the  Black  Oak  mine,  and  the 
proportion  is  2  or  3  parts  of  silver,  by  weight,  to  one  of  gold.  In  this 
proportion  the  gold  is,  of  course,  by  far  the  most  important  factor  to  be 
considered  in  the  metallurgy  of  the  ore,  but,  as  will  be  mentioned  later, 
the  silver  is  in  sufficient  quantity  to  make  advisable  a  special  effort  to 
recover  a  goodly  proportion  of  it. 

Most  of  the  valuable  metal  is  in  quartz  which  occurs  in  a  granite-diorite 
country  rock,  and  sulphides  contain  the  greater  part  of  the  silver  and  gold. 
As  often  happens  in  California  ores,  this  gold  does  not  seem  to  be  chemi- 
cally combined  with  the  sulphides,  but  only  mechanically,  so  that  fine 
grinding  liberates  the  greater  part  of  it.  The  silver  is,  of  course,  largely  in 
combination  as  sulphide,  in  which  condition  it  is  amenable  to  cyanidation 
after  having  been  subjected  to  fine  grinding. 

Comparison  with  Other  Ores. — It  is  noteworthy  that  the  ore  deposits 
of  the  Black  Oak  mine,  and  of  the  surrounding  district  similar  in  a  general 
way  to  the  mineral  already  described  at  Grass  Valley,  are  said  to  be  similar 
to  the  majority  of  ores  in  the  mines  on  the  Mother  Lode,  and  identical 
with  some  of  them.  If  this  is  true,  the  question  immediately  is  suggested 
as  to  why  the  other  Mother  Lode  mines,  or  at  least  some  of  them,  do  not 
make  use  of  cyanidation,  a  process  which  has  demonstrated  its  efficiency 
and  economy  on  similar  material.  Mother  Lode  ores  are  still  being 
beneficiated  by  old  methods,  amalgamation  and  concentration.  While 
both  of  these  systems  are  simple  and  cheap,  it  is  undeniable  that  neither  of 
them  approaches  cyanidation  in  the  amount  of  recovery  from  simple  ores. 
Some  have  informed  me  that  Mother  Lode  ores  are  not  treatable  by  cyani- 
dation, while  others  have  assured  me  quite  as  emphatically  that  they  are. 

188 


THE  BLACK  OAK  PLANT,  CALIFORNIA 


189 


190  DETAILS  OF  CYANIDE  PRACTICE 

In  view  of  results  obtained  at  Black  Oak  and  Grass  Valley,  the  preponder- 
ance of  evidence  would  seem  to  be  in  favor  of  the  latter  statement.  Per- 
haps the  experience  of  these  two  camps  will  be  of  value  to  those  opera- 
tors on  the  Mother  Lode  who  desire  to  increase  the  efficiency  of  their 
installations. 

The  Crushing  System. — Ore  from  the  mine  is  first  delivered  to  the  mill 
bin-  which  has  a  capacity  of  about  185  tons.  Four  Challenge  feeders 
deliver  it  into  the  mortars  of  a  20-stamp  crushing  plant,  each  stamp  weigh- 
ing 1250  Ib.  and  dropping  102  times  per  minute  through  6  in.  Mortars 
are  of  the  narrow  pattern  designed  for  rapid  crushing.  Three  of  them  are 
equipped  with  ton-cap  screens  having  19  apertures  per  inch,  and  the  re- 
maining mortar  has  a  6-mesh,  square  aperture  screen.  This  coarse  screen 
adds  sufficient  granular  material  to  increase  the  efficiency  of  subsequent 
tube  mill  work.  Many  of  the  recently  constructed  mills  are  adopting  the 
system  of  varied  feed  for  tube  mills  and  the  results  thus  far  seem  to  have 
amply  justified  the  practice. 

The  Black  Oak  mill  was  originally  a  40-stamp  plant,  containing  the 
old-style  stamps  of  light  weight,  but  it  has  been  entirely  rebuilt  with  new 
and  appropriate  machinery.  There  is  still  room  for  40  stamps  under  the 
present  room,  in  fact  20  of  the  original  stamps  are  still  there  in  a  more  or 
less  dismantled  condition.  With  further  mine  development  all  available 
space  may  be  required. 

Classifying  and  Regrinding. — Pulp  from  the  stamps  flows  by  gravity 
to  a  Dorr  classifier  of  the  usual  type  in  which  the  slime  is  separated  and 
taken  directly  to  treatment  and  the  sand  is  delivered  to  a  tube  mill 
for  further  grinding.  This  tube  mill  is  5  X 18  ft.  and  is  of  the  type  made 
by  the  Power  &  Mining  Machinery  Co.  It  is  operated  at  26  r.p.m.  and 
requires  48  hp.  to  keep  it  moving.  Imported  pebbles  are  used,  the  con- 
sumption of  which  is  5  Ib.  per  ton  of  ore  crushed.  The  hardness  of  the 
ore  is  attested  by  this  consumption  of  pebbles  which  is  extraordinarily 
high. 

Ground  product  from  the  tube  mill  is  returned  to  the  classifier  by 
means  of  a  bucket  elevator,  thus  forming  a  closed  circuit,  the  only  exit 
from  which  is  the  slime  in  condition  for  agitation  treatment. 

Upon  its  exit  from  the  classifier,  the  slime  proceeds  to  a  10X20-ft. 
Dorr  thickener,  where  a  portion  of  the  solution  is  decanted  and  a  thick- 
ened pulp  delivered  of  proper  consistency  for  the  subsequent  cyanidation. 

Agitation  of  the  Slime. — Slime  treatment  is  performed  in  tanks  of  the 
Pachuca  type  of  which  there  are  three,  each  10X30  ft.  Passage  of  the 
pulp  is  continuous  through  these  tanks  and  during  the  time  of  agitation 
the  extraction  of  the  contained  gold  is  practically  complete. 

The  Dorr  thickener,  which  prepares  the  pulp  for  the  agitation  tanks, 
delivers  a  pulp  in  which  the  proportion  of  slime  to  solution  is  about  1 : 1 J. 


THE  BLACK  OAK  PLANT,  CALIFORNIA 


191 


The  thickener  is  operated  at  about  one  revolution  in  six  minutes,  and  in 
common  with  these  machines,  requires  an  extremely  small  expenditure  of 
power. 

185 -Jon  Ore  Bin 
4-Challenge  Feeders 

20- 1250 1  b.  Stamps,  narrow 
Mortars,  102  Drops.-  6  "drop 

I-  Duplex  Dorr  Classifier 


/- 5x18 '  Tube  Mill,  26  R.p.m. 
Bucket  Elevator 

I- 10x20' Dorr  Thickener 

3-10x30  Pachuca-Type  Agitators 
(Continuous  System) 

I-10x20'Dorr  Thickener 

H2' Oliver  Filter 
Slime  Residue 


\ 


Solution  Collecting  Tank 

2 ~5 6  Frame, 24 "Clarify 'ing  Presses 
Pregnant  Solution  Tank 
Zinc  Feeder 

1-16  Frame,  52"t1er  rill  Precipitation 

Press 
~t- 
-^.     Solution  Sump 

Drying  Pan 

Oil  Fired  Tilting  Furnace 


Bullion 
FLOW  SHEET  OF  BLACK  OAK  MILL. 


The  material  agitated  is  true  slime,  or  colloid,  only  in  small  proportion, 
the  major  part  consisting  of  extremely  fine  granular  matter.     About  78% 

\ 


192  DETAILS  OF  CYANIDE  PRACTICE 

of  it  is  sufficiently  fine  to  pass  a  200-mesh  screen,  while  98  to  99%  will 
pass  a  150-mesh  aperture.  Pipe  connections  for  the  continuous  passage 
of  the  pulp  are  4  in.  in  diameter. 

Continuous  Filtration. — Issuing  from  the  agitation  treatment  in  the 
Pachuca  tanks,  the  pulp  is  received  in  a  second  Dorr  thickener,  identical 
in  its  dimensions  with  the  one  first  mentioned.  Just  before  reaching  this 
thickener,  the  pulp  is  diluted  with  the  overflow  product  of  the  first 
thickener  and  the  mixture  thickened  to  a  point  where  the  underflow  of 
pulp  issuing  contains  solution  and  solid  in  a  1 : 1  proportion. 

This  thickened  pulp  is  delivered  to  a  12-ft.  Oliver  continuous  filter. 
The  slime  residue  is  dewatered  and  discarded  and  the  solution,  together 
with  the  overflow  from  the  final  Dorr  thickener,  is  gathered  in  a  collecting 
tank  from  which  it  is  taken  to  the  precipitation  system. 

For  precipitation  the  Merrill  system,  using  zinc  dust,  is  followed.  A 
special  zinc-dust  feeder  drops  the  precipitant  into  the  stream  of  pregnant 
solution  going  from  the  tank  to  the  suction  of  the  pump,  and  the  mixture 
is  forced  through  al6-frame  filter  press  of  the  Merrill  type.  The  frames  of 
this  press  are  52  in.  across  the  top  and  are  triangular  in  shape  as  is  usual. 

Solution,  after  precipitation,  is  received  in  a  sump  from  which  it  is 
pumped  up  to  the  solution  storage  tank  for  further  use  in  the  mill. 

Precipitate  from  the  press  is  partially  dried  in  a  drying  pan,  fluxed  and 
melted  in  a  tilting,  oil-fired  furnace.  The  accompanying  flow-sheet  dia- 
gram illustrates  the  metallurgical  process  followed  in  the  mill  and  the 
principal  machinery  installed. 

Character  of  the  Dry  Slime. — The  Black  Oak  ore  is  hard,  as  has 
already  been  mentioned,  and  remains  largely  in  the  granular  state  even 
after  continued  grinding.  The  specific  gravity  of  the  dry  slime  in  the 
condition  in  which  it  is  subjected  to  agitation  treatment,  is  about  2.8,  a 
figure  which  is  rather  high  in  comparison  with  ordinary  siliceous  slimes, 
which  generally  contain  a  greater  proportion  of  colloid  matter. 

The  mill  will  treat  about  100  tons  per  day  of  24  hours,  but  at  present 
the  rate  of  crushing  is  near  85  tons  daily.  Average  run  of  ore  will  contain 
a  combined  gold  and  silver  value  of  $15  per  ton  in  the  proportions  which 
have  already  been  mentioned. 

Of  the  gold,  97%  is  usually  extracted  without  difficulty,  and  about 
92%  of  the  silver.  Treatment  solutions  contain  0.2%  KCN  and  0.15% 
CaO.  Lime  is  added  at  the  batteries  and  in  the  agitators.  Litharge  is 
used  as  a  source  of  lead  for  removing  soluble  sulphides  from  solution  and 
is  added  at  the  tube  mill  in  order  to  facilitate  solution. 

Chemical  Consumption. — The  consumption  of  chemicals  and  supplies 
is  low,  as  is  usual  when  treating  an  ore  whose  value  is  largely  in  its  gold 
content.  The  cyanide  consumption  is  1.0  Ib.  per  ton;  of  litharge,  0.35  lb.; 
of  lime,  3  lb.;  of  zinc,  1  lb.,  and  of  pebbles,  5  lb.  per  ton. 


THE  BLACK  OAK  PLANT,  CALIFORNIA  193 

TABLE  OF  COSTS  AT  THE  BLACK  OAK  MILL,  CALIF.,  BASED  ON  MILLING 

2202.5  TONS 

Supplies  Per  Ton 

Rock  crusher $0.0121 

Batteries 0.0158 

Tube  milling 0.0349 

Tube-mill  liners 0.0266 

Classification 0.0039 

Refining 0.0195 

Repairs 0.0131 

Office  and  general  superintendent 0 . 0042 

Assaying 0.0241 

Marketing  bullion: 

Control $10.45 

Treatment.  .  .   167.38 

Express 25.25 0.0922 

Cyanide 0.2018 

Lime 0.0111 

Litharge 0 . 0507 

Zinc 0.0970 

Pebbles..  0.0780  $0.6850 


Power  and  lights $0. 3500 

Insurance 0.0200 

Labor 

Rock  crusher. $0.0453 

Tramming 0.0844 

Batteries 0 . 0968 

Tube  milling 0.0184 

Classification 0. 0192 

Agitation 0. 0223 

Thickening 0.0223 

Pumping  and  elevating 0. 0244 

Filtration 0 . 0246 

Clarification 0.0224 

Precipitation 0.0236 

Refining 0.0305 

Repairs 0.0523 

Office  and  general  superintendent. 0.0715 

Mill  superintendent 0.0681 

Power  and  light 0.0165 

Assaying 0.0315     $0.6741 


Total  mill  costs $1 .7291 

Seven  men,  excluding  the  superintendent,  are  required  for  mill  opera- 
tion, and  130  hp.  is  the  average  consumption  of  energy  for  moving  the 
machinery.  While  the  mill  is  small  and  handles  only  a  limited  quantity 
of  ore,  costs  have  been  reduced  to  an  extremely  reasonable  figure  as  is 


194  DETAILS  OF  CYANIDE  PRACTICE 

shown  by  the  accompanying  tabulated  list  of  items.  During  the  two 
months  following  that  in  which  this  report  was  made,  a  general  reduction 
of  costs  has  been  accomplished,  the  total  being  reduced  to  about  $1.61 
per  ton.  The  operators  are  fortunate  in  having  an  ample  supply  of  water 
and  conditions  generally  favorable  for  successful  operation. 


CHAPTER  XVIII 
THE  GOLD  ROAD  MILL,  ARIZONA 

The  mill  of  the  Gold  Road  Mines  Co.  is  situated  at  Gold  Road,  about 
20  miles  from  the  town  of  Kingman  in  western  Arizona.  Railroad  com- 
munication is  established  at  Kingman  and  connection  with  the  mine  is  by 
wagon  road  over  which  freight  is  hauled  by  horses,  and  passengers  by 
automobile  stage.  The  region  is  arid,  water  is  scarce  and  has  to  be  con- 
served with  care. 

Local  ores  have  gold  as  the  principal  valuable  constituent,  the  metal 
being  finely  divided  and  distributed  through  a  gangue  consisting  of  quartz 


GOLD  ROAD  MILL,  GOLD  ROAD,  ARIZ.   LOOKING  TOWARD  THE  CRUSHING  PLANT. 

and  some  calcite  in  a  country  of  andesitic  rock.  There  is  no  great  quan- 
tity of  sulphides,  the  gold  being  free  and  principally  fine.  On  account  of 
the  fineness  and  wide  distribution  of  the  gold,  a  rather  high  degree  of 
crushing  is  required  and  the  total-sliming  process  is  followed. 

The  mill  is  not  an  original  construction,  but  rather  a  development,  the 
remnants  of  machinery  used  in  various  processes,  now  relinquished,  remain 
as  reminders  of  the  older  systems  of  metallurgy.  This  changing  of  sys- 
tems has  also  resulted  in  a  scattering  of  the  reduction  plant  over  a  good 

195 


196 


DETAILS  OF  CYANIDE  PRACTICE 


deal  of  territory,  a  fact  which  may  be  seen  by  a  study  of  the  accompany- 
ing engravings.  Notwithstanding  this  circumstance,  a  plant  of  ex- 
ceedingly good  metallurgical  performance  is  now  in  operation,  the  details 
of  which  are  explained  in  this  paper. 

Preliminary  Breaking. — Installed  at  the  mine  is  a  No.  5  Gates  gyratory 
crusher  where  the  ore  receives  its  preliminary  breaking.  From  the 
crusher  it  is  trammed  to  the  mill,  where  it  is  delivered  into  a  1540-ton  bin 
which  supplies  the  mill  as  required.  During  the  preliminary  breaking  at 
the  mine,  the  required  portion  of  lime  is  added  to  the  ore  so  that  by  the 
time  the  material  has  passed  through  the  bin  and  entered  the  stamp-crush- 


GOLD    ROAD    MILL,    GOLD    ROAD,    ARIZ.       UPPER    SIDE   VIEW. 

ing  department,  the  lime  has  performed  a  great  deal  of  its  essential  work, 
neutralizing  the  developed  acidity  and  becoming  more  or  less  slaked  and 
ready  to  offset  the  acids  which  may  be  formed  later  in  the  course  of  its 
treatment.  This  method  of  lime  addition  is  advocated  by  many  operators 
because  of  the  advantages  enumerated  above,  and  further,  because  the 
lump  form  is  progressively  crushed  as  is  the  ore,  and  develops  alkaline 
conditions,  when  the  addition  is  at  the  appropriate  rate,  just  as  the  ore 
develops  acidity.  Others  advocate  the  powdered  form  of  lime,  sometimes 
previously  slaked,  and  make  additions  at  the  point  and  time  when  acidity 
develops.  No  doubt  a  greater  percentage  of  CaO  can  be  dissolved  by 
careful  slaking  under  favorable  conditions,  but  whether  the  highest  ulti- 
mate economy  and  efficiency  are  reached  by  so  doing,  is  a  question  which 
will  have  to  be  solved  for  every  problem  after  consideration  of  all  the 
conditions. 


THE  GOLD  ROAD  MILL,  ARIZONA  197 

Crushing  and  Grinding. — From  the  bin,  Challenge  feeders  deliver 
the  ore  into  a  battery  of  forty  1050-lb.  stamps,  making  104  drops  per  min. 
through  7  in.  The  stamp  crushing  is  done  in  solution.  Screens  of  4- 
mesh  are  used  on  the  batteries,  the  stamp  duty  being  about  nine  tons  per 
stamp  per  day,  or  a  total  of  3"60  tons  daily  for  the  mill. 

Pulp  from  the  stamps  is  led  to  two  distributing  cones,  one  4  ft.  and 
one  6  ft.  in  diameter,  from  which  it  goes  to  six  duplex  Dorr  classifiers, 
where  the  overflow  slime  is  removed  and  the  sand  dewatered  and  delivered 
to  the  tube  mills  for  regrinding. 

There  are  four  tube  mills,  each  5X22  ft,  used  in  closed  circuit  with 
the  classifiers,  the  delivery  from  the  tube  mills  being  elevated  with  10X54- 
in.  Frenier  pumps  and  returned  to  the  classifiers  where  any  sand  not 
sufficiently  fine  for  treatment  by  agitation,  is  removed  and  returned  to  the 
tube  mill  for  further  grinding.  El  Oro  linings  are  used  in  these  mills  and 
are  said  to  be  entirely  satisfactory,  each  lining  lasting  a  year  before  it  is 
replaced.  Imported  pebbles  are  used  in  the  mills,  the  consumption  being 
4  Ib.  per  ton  crushed. 

It  has  been  practically  found  unnecessary  to  grind  this  ore  through 
a  200-mesh  screen,  the  product  passing  a  100-mesh  screen  giving  as  good 
extraction  results.  In  practice,  the  pulp  delivered  to  the  agitation  treat- 
ment contains  about  86  to  87%  which  will  pass  a  150-mesh  screen.  The 
specific  gravity  of  the  dry  slime  has  been  determined  and  is  stated  to 
be  2.63,  a  figure  denoting  about  medium  weight. 

Thickening  and  Agitation. — The  entire  pulp  from  the  grinding  system 
is  received  in  two  30  X  10-ft.  Dorr  thickeners,  the  overflow  solution  going 
to  clarifying  tanks  and  the  underflow  of  thickened  pulp  being  taken  to  the 
agitation  system.  This  agitation  installation  consists  of  three  17X44-ft. 
Pachuca  tanks  which  were  used  in  series,  but,  having  found  that  so  long 
a  time  for  agitation  was  not  necessary  to  secure  highest  extraction,  this, 
together  with  the  inherent  inconveniences  of  this  kind  of  tank,  resulted 
in  cutting  out  two  of  them  and  the  using  of  only  one. 

With  the  pulp  of  the  grade  and  character  customary  at  Gold  Road,  it  is 
somewhat  difficult  to  prevent  settling  of  a  portion  on  the  solids,  and  in  such 
cases  the  Pachuca  tank  has  often  been  found  unsatisfactory.  Here  the 
settlement  became  so  great  that  the  time  of  agitation  was  reduced  to 
practically  nothing.  It  is  stated  that  the  settlement  was  so  hard  and  solid 
that  one  could  step  over  the  edge  into  the  tank  and  walk  about,  keeping 
away  from  the  center,  without  any  inconvenience. 

Thickened  pulp,  that  coming  from  the  thickeners  into  the  agitation 
department,  has  the  dilution  ratio  of  about  1  to  1,  and  retains  this  con- 
sistency until  after  finishing  the  agitation  treatment.  Contact  between 
the  ore  and  cyanide  solution  was  36  to  48  hr.  in  the  mill,  counting  from  its 
first  pulping  at  the  cones,  to  its  discharge.  Owing  to  a  change  in  the 


198 


DETAILS  OF  CYANIDE  PRACTICE 


metallurgical  system,  this  time  of  contact  may  have  become  somewhat 
protracted,  but  not  because  of  any  necessity  of  increasing  the  per  cent,  ex- 


THE  GOLD  ROAD  MILL,  AGITATION  AND  FILTRATION  PLANT. 


THE  GOLD  ROAD  MILL,  THICKENERS  AND  SOLUTION  TANKS. 

traction.     The  greater  part  of  the  extraction  is  secured  before  the  agita- 
tion treatment,  and  practically  none  after  it. 

Issuing  from  the  agitation  system  in  the  Pachuca  tanks,  the  pulp  is 


THE  GOLD  ROAD  MILL,  ARIZONA  199 

delivered  to  a  series  of  thickeners  where  the  pulp  is  successively  dewatered 
and  rediluted,  thus  washing  out  the  dissolved  metal  in  the  pulp. 

This  treatment  is  not  that  which  has  been  generally  followed  at  the 
Gold  Road  mill.  The  former  method,  followed  until  a  few  months  ago, 
involved  delivering  the  slime  from  the  agitation  system  into  a  slime-storage 
tank  whence  it  was  taken  by  a  120-leaf  Butters  filter  installation  and 
handled  in  the  usual  way.  After  the  cake  had  been  dropped  from  the 
filter  leaves,  it  was  mixed  thoroughly  in  a  tank  containing  mechanical 
agitators  and  discharged  by  gravity  after  passing  an  automatic  mechanical 
sampler. 

For  many  reasons,  this  system  has  been  dispensed  with  and  the  pulp 
from  the  agitation  system  is  delivered  to  two  thickeners. 

Filtration  vs.  Decantation. — The  solution  overflow  from  these  thick- 
eners is  pumped  back  to  the  head  of  the  mill  and  used  as  mill  solution, 
while  the  thickened  underflow  goes  to  two  more  thickeners,  before  reach- 
ing which,  however,  it  is  diluted  with  solution  previously  precipitated. 
The  solution  overflow  from  this  second  pair  of  thickeners  is  sent  back  to 
dilute  the  pulp  entering  the  first  pair  of  thickeners.  This  solution,  over- 
flowing from  the  first  pair  of  thickeners,  does  not  require  immediate  pre- 
cipitation as  it  consists  largely  of  precipitated  solution  as  has  been  shown, 
The  first  solution  for  precipitation  is  taken  from  the  overflow  of  the  Dorr 
thickener,  through  which  the  pulp  passes  before  reaching  the  agitation 
system  in  the  Pachuca  tanks. 

Second  Thickening  System. — As  a  final  result,  then,  the  pulp  from  the 
agitation  system  going  into  the  first  thickening  unit,  consisting  of  two 
pairs  of  thickeners,  issues  from  it  in  form  of  a  thick  pulp  of  about  the  same 
consistency  at  which  it  entered,  having  been  meantime  diluted  with 
barren  solution,  and  twice  thickened.  The  barren  solution  has  passed 
through  the  pulp,  diluted  the  metal-bearing  solution,  and  carried  most  of 
its  value  to  the  mill  solution  circulation.  From  this  circulation  a  great 
proportion  of  the  solution  passes  to  precipitation  from  the  first  Dorr 
thickener,  which  follows  the  tube  mills,  avoiding  any  extensive  building 
up  of  value  in  solution. 

Underflowing  from  the  first  thickening  system,  the  thick  pulp  goes  to  a 
second  system,  which  consists  of  three  pairs  of  thickening  tanks.  In  this 
unit  the  pulp  is  repeatedly  thickened  and  washed , water  in  this  case  being 
the  washing  medium.  The  counter-current  system  is  followed,  water 
entering  at  the  final  thickener  and  progressing  backward  to  the  first  of  the 
series,  the  solids  taking  the  opposite  direction. 

There  is  provided  a  separate  precipitation  circuit  to  accompany  this 
washing  unit,  with  facilities  for  diverting  the  solution  either  into  the  mill- 
solution  circuit,  before  precipitation,  or  into  the  washing  circuit  itself 
after  precipitation.  The  precipitate  joins  the  bulk  of  that  from  the  main 


200  DETAILS  OF  CYANIDE  PRACTICE 

precipitation  system,  which  is  carried  to  the  refining  department.  The 
movement  of  pulp  and  solution  is  graphically  shown  in  the  accompanying 
flow  sheet. 

Preference  for  Decantation. — Undoubtedly,  the  adoption  of  this 
system  will  be  regarded  with  interest  by  everyone  connected  with  the 
process.  Not  merely  is  it  interesting  as  an  example  of  continuous  coun- 
ter-current decantation,  noteworthy  as  it  is  from  that  point  of  view,  but 
more  particularly  because  that  system  has  supplanted  a  large,  modern, 
vacuum-filter  installation  with  results,  according  to  statement  of  the 
operators,  better  than  those  obtained  under  former  method  of  operation. 

There  is  food  for  considerable  serious  thought  in  this  change,  but  there 
is  danger  of  falling  into  popular  form  of  error,  and  regarding  the  procedure 
as  a  reversion  to  a  system  already  discarded,  or,  in  other  words,  a  step 
backward.  A  little  thought,  however,  will  make  clear  the  fact  that  it  is 
nothing  of  the  kind,  but  rather  a  refinement  of  an  earlier  process,  than 
which  no  change  can  be  more  progressive.  It  is  clear  to  the  most  casual 
observer  that  the  system  used  at  the  Gold  Road  mill  is  not  applicable  to 
all  mills,  although  it  may,  as  in  the  present  instance,  be  applied  occasion- 
ally with  favorable  results  both  as  to  percent  of  recovery  and  cost  of 
operation.  It  is  applicable  in  its  present  form  only  where  solutions  carry- 
ing low  cyanide  percentages  can  be  used  and  where  the  amount  of  metal 
in  solution  is  comparatively  small,  conditions  which  are  manifestly  present 
when  low  or  medium-grade  gold  ores  are  treated.  Where  high-grade 
solutions  are  used,  it  would  be  impossible  to  use  the  system  in  this  form 
because  a  large  loss  of  cyanide  would  be  entailed,  but  by  following  up 
the  decantation  with  a  form  of  continuous  filtration  which  would  reduce 
the  moisture  content  of  the  residue  to  about  25%,  the  system  is  not  only 
feasible,  but  advantageous.  To  steer  clear  of  generalities  is  necessary  for 
the  intelligent  consideration  of  any  innovation,  so  that  in  estimating  the 
value  of  this  one,  it  must  not  be  considered  revolutionary  nor  reactionary, 
nor  anything  more  than  a  simple,  clear  application  of  a  newly  developed 
system  to  a  particular  case. 

Precipitation  by  Zinc  Dust. — Solutions  of  both  circuits  are  precipi- 
tated with  zinc  dust,  the  special  feeders  and  presses  being  provided  in  each 
case,  as  is  shown  in  the  flowsheet.  Square-frame  presses  are  used,  and  not 
the  triangular  form  usually  found  in  Merrill  installations.  A  variation  of 
the  general  filter-press  practice  is  the  use  of  three  filter  coverings  for  each 
plate.  The  layer  next  the  cake  of  precipitate  is  of  filter  paper,  the  inter- 
mediate one  of  light  muslin,  and  the  outer  one  of  the  usual  canvas.  The 
paper  is  removed  and  burned  with  each  precipitate  cake,  the  intermediate 
sheet  lasts  for  several  clean-ups,  while  the  canvas,  by  careful  cleaning,  is 
made  to  last  indefinitely. 

The  method  of  adding  zinc  dust  to  the  solution  is  the  Bosqui  system, 


THE  GOLD  ROAD  MILL,  ARIZONA 


201 


Water  Storage  Tanks^/^-f-, 

C\Tanks 


No.  5  Gates  Crushers 

atM'-ne 
Tram  -to  Bin 

1540  Ton  Ore  Bin 

8  Challenge  feeders 

104  brops.+Mesh  Screens 


^-Clarifying  Tanks 
Clarifying  Press      His:!  fc    ^ 


Clarifying  Press    '5 
Zinc  feeder 


Precipi-ra-re 
'  Press 


Tilting  furnace 


Bullion 
FLOW  SHEET  OF  GOLD  ROAD  MILL. 


14 


202  DETAILS  OF  CYANIDE  PRACTICE 

controlled  by  the  Merrill  Metallurgical  Co.  and,  as  is  usual,  the  Trent 
agitator  is  used  for  emulsifying.  The  consumption  of  zinc  dust  amounts 
to  about  0.4  Ib.  per  ton  of  solution  precipitated. 

Precipitate  is  handled  in  the  usual  way  Formerly  acid  treatment  was 
not  resorted  to,  but  at  present  a  preliminary  acid  "  cutting-down"  is 
made  and  the  mass  of  precipitate  is  then  roasted.  The  dry  product  of  the 
roasting  pan  is  fluxed  and  melted  in  a  Steele-Harvey  oil-fired  furnace. 

Experience  at  this  plant  has  been  such,  as  to  demonstrate  that  solutions 
containing  exceptionally  low  percentages  of  cyanide  can  be  precipitated 
successfully  when  the  percentage  of  lime  carried  is  sufficiently  high.  Solu- 
tions containing  as  little  as  2c.  to  15c.  in  gold  and  very  weak  in  cyanide, 
solutions  from  washing  systems,  are  successfully  precipitated  by  carrying 
high  alkalinity. 

As  has  already  been  mentioned,  the  lime  is  added  at  the  mine  crushing 
plant.  The  consumption  amounts  to  about  0.65  Ib.  per  ton  treated. 
Treatment  solutions  usually  carry  2  to  2J  Ib.  CaO,  per  ton  of  solution. 

Cyanide  in  the  Tube  Mill. — The  regular  addition  of  cyanide  for 
keeping  up  the  strength  of  mill  solutions,  is  at  the  tube  mill.  This 
is  often  considered  an  excellent  point  for  addition  of  cyanide,  as  it  is 
immediately  crushed  and  put  into  solution;  besides,  the  energetic  agita- 
tion in  the  mill,  together  with  the  somewhat  elevated  temperature,  gives 
the  best  opportunity  for  dissolving  the  greatest  possible  quantity  of  metal. 

Solutions,  at  the  Gold  Road  mill,  are  carried  at  about  2  to  2J  Ib.  KCN 
per  ton,  only  one  strength  being  in  use.  The  consumption  is  about  0.4  Ib. 
per  ton  of  ore  treated. 

Extraction  of  gold  is  usually  about  94%  or  more,  and  is  calculated  by 
using  the  combined  content  of  bullion  produced  and  tailing  discharged  as 
the  content  of  the  ore  milled.  Recognizing  the  limitations  of  this  system, 
careful  duplication  and  check  samples  are  taken  continuously  in  various 
parts  of  the  mill. 

The  total  milling  process  requires  530  hp.  for  its  operation  and  20 
men  are  employed  continuously,  two  of  these  being  repair  men. 

Detailed  cost  data  on  this  installation  would  be  exceedingly  inter- 
esting, but  unfortunately  the  policy  of  the  company  is  adverse  to  such 
publication,  and  for  that  reason  no  figures  can  be  given.  It  may,  how- 
ever, be  said  that  the  expenses  are  altogether  reasonable  and  in  conformity 
with  those  found  satisfactory  in  the  best  installations  of  its  kind. 


CHAPTER  XIX 
TWO  ARIZONA  MILLS 

The  property  of  the  Tom  Reed  Gold  Mines  Co.  is  situated  in  the 
western  part  of  the  state  of  Arizona,  not  far  from  the  California  line.  The 
village  of  Oatman,  which  consists  principally  of  the  officers  and  employees 
of  the  Tom  Reed  Co.,  is  not  reached  by  the  railroad,  but  communication 
is  established  at  Kingman,  about  20  miles  distant.  With  Kingman, 


GENERAL  VIEW  OF  TOM  REED  MILL,   OATMAN,  ARIZ. 

transportation  is  by  means  of  automobile  stage  for  passengers;  machinery 
and  supplies  are  brought  in  on  wagons  and  occasionally  pack  animals. 

The  situation  is  in  a  rocky,  desert  country,  comparatively  isolated,  the 
nearest  activity  being  at  the  Gold  Road  mine  of  the  U.  S.  Smelting,  Re- 
fining &  Mining  Co.,  about  1|  miles  distant.  Aside  from  this  large  mine, 
there  is  nothing  within  a  much  greater  distance.  Water  is  scarce  and  its 
use  has  to  be  governed  by  great  economy.  The  name  of  the  company^is 
taken  from  the  name  of  one  of  the  claims  which  comprise  the  property. 

203 


204 


DETAILS  OF  CYANIDE  PRACTICE 


Character  of  the  Ore. — Gold  is  the  principal  valuable  metal  in  the  ore, 
and  it  occurs  generally  free  in  quartz  and,  to  a  smaller  extent  in  calcite. 
The  ore  is  not  particularly  hard,  but  requires  fairly  fine  grinding  to  free 
the  gold  and  expose  it  to  the  action  of  cyanide  solutions. 

From  $15  to  $25  per  ton  is  the  general  variation  in  the  value  of  the  ore 
treated  in  the  mill  and  the  average  for  several  months  at  a  time  has  been 
near  the  higher  figure.  While  there  is  a  small  amount  of  silver  occurring 
with  the  gold,  it  is  not  significant  and  no  effort  to  save  it  would  be  worth 
while.  At  present  the  mill  treats  about  150  tons  daily  of  dry  ore,  crush- 
ing with  stamps  and  regrinding  with  tube  mills.  The  total-sliming  system 


FILTER  PLANT  DISCHARGE  AT  TOM  REED  MILL. 

of  cyanidation  is  followed,  crushing  in  cyanide  solution  containing  2  Ib. 
KCN  per  ton.  At  the  point  where  it  is  ready  for  the  agitation  treatment, 
the  solids  in  the  pulp  will  average  about  70%  through  a  200-mesh  screen 
and  remaining  on  100  mesh  the  quantity  is  about  1  to  2%,  the  whole 
making  a  product  not  particularly  difficult  to  agitate. 

Character  of  Material. — While  pulp  of  this  class  may  be  difficult  to 
agitate  in  those  cases  when  it  is  largely  heavy,  granular  and  devoid  of 
colloid  constituent,  the  governing  factor  is  usually,  or  ought  to  be,  the 
fineness  necessary  to  liberate  the  economically  maximum  amount  of  gold. 
The  question  of  fine  grinding  has  prompted  vigorous  partisanship  for  or 
against  it  by  those  metallurgists  who  believe  strongly  in  extremes  of  fine 
grinding  or  no  regrinding  at  all.  Some  maintain  that  extremely  fine 


TWO  ARIZONA  MILLS  205 

grinding  is  wasteful  and  costly,  and  that  if  the  grinding  is  not  carried  to 
extremes,  agitation  of  the  entire  pulp  is  unnecessary  and  extravagant. 
Certainly  there  are  several  factors  which  must  be  given  consideration. 
Primarily,  of  course,  the  cost  of  fine  grinding  must  be  weighed  against  the 
additional  income  furnished  by  it,  and,  to  be  truly  profitable,  this 
increased  income  must  be  sufficient  to  cover  the  cost  of  regrinding,  interest 
and  depreciation  on  the  grinding  plant,  and  leave  a  profit  in  addition. 
Even  this  comparison  alone,  however,  is  not  sufficient  to  justify  a  proper 
decision.  Part  of  the  material  reduced  to  the  so-called  state  of  slime  is 
likely  to  be  granular,  though  fine,  and  its  agitation  will  cost  more  than  if 
a  true  slime,  or  colloid  matter,  were  being  handled.  This  extra  cost  of 
agitation  is  not  to  be  ignored  in  an  exhaustive  search  for  definite  facts. 

Contrary  to  what  seems  to  be  the  general  belief  a  large  proportion  of 
this  fine  granular  material  might  be  successfully  leached  provided  only 
that  it  is  well  cleaned,  that  is,  entirely  freed  of  colloid  material.  If, 
and  when,  this  granular  material  is  sufficiently  fine  to  liberate  the  con- 
tained metal  and  expose  it  to  the  action  of  cyanide  solutions,  it  is  more 
than  likely  that  leaching  will  provide  for  recovery  of  an  amount  equal  to 
that  which  could  be  obtained  by  agitation,  at  much  less  cost.  Leach- 
ing will  take  longer  to  accomplish  the  same  result,  but  the  cost  will  be 
less  and  extraction  will  be  as  good  in  many  cases.  Also,  the  installation 
of  a  leaching  plant  costs  less  than  that  for  agitation,  and  maintenance 
expenses  and  interest  are  therefore  lower.  In  calculating  the  cost  of 
leaching  plants,  however,  it  should  not  be  forgotten  that  the  separation, 
meaning  a  definite  and  clearly  marked  separation,  of  fine,  granular  mater- 
ial from  a  slime  pulp  is  not  a  simple  operation  and  is  likely  to  cost  an 
appreciable  sum  per  ton  treated.  The  most  natural  deduction  is  that 
every  problem  should  receive  separate  study  and  be  solved  according  to  its 
requirements.  At  the  Tom  Reed  plant,  the  grinding  is  carried  to  a  fine 
point  where  the  product  is  comparatively  easy  to  agitate  and  gives 
better  extraction;  hence  the  total-sliming  treatment. 

Stamp -crushing. — At  the  Tom  Reed  mine  the  ore  is  hoisted  and  passed 
over  a  grizzly  and  through  a  No.  2  Austin  gyratory  crusher.  Thus  pre- 
pared, the  ore  is  delivered  to  a  long  16-in.  conveyor  belt  which  delivers  it 
to  the  mill  bin.  From  the  bin,  Challenge  feeders  supply  a  battery  of  20 
stamps  of  1250  Ib.  each,  dropping  105  times  per  minute  through  6  in. 
The  material  is  crushed  to  pass  a  12-mesh  screen.  Crushing  is  done  in 
cyanide  solution  containing  2  Ib.  KCN  per  ton  and  alkalinity  equal  to 
about  2  Ib.  CaO  per  ton  of  solution  is  carried. 

From  the  stamps  the  pulp  is  received  in  a  6-ft.  hydraulic  cone,  the 
slime  overflowing  to  a  circuit  which  will  be  described,  and  the  sand  going 
directly  into  the  first  tube  mill. 

Tube -milling  and  Classification. — Sand,  or  underflow,  from  the  6-ft. 


206  DETAILS  OF  CYANIDE  PRACTICE 

cone  is  fed  into  the  first  tube  mill  which  is  5X22  ft.,  and  equipped  with 
El  Oro  lining.  From  this  mill  the  entire  pulp  is  delivered  into  a  Dorr 
duplex  classifier,  from  which  the  slime  overflow  joins  the  overflow  from 
the  6-ft.  cone  first  mentioned  and  the  sand  is  delivered  to  an  8X54-in. 
Frenier  pump,  and  thence  to  the  second  tube  mill.  This  mill  is  a 
small  one,  3|X  16  ft.,  and  is  also  fitted  with  the  El  Oro  lining,  which  has 
given  good  results  at  this  mill.  The  pebble  consumption  in  these  mills  is 
4  Ib.  per  ton  of  ore  crushed.  This  small  tube  mill  delivers  its  product  into 
another  hydraulic  cone,  this  one  5  ft.  in  diameter,  whose  overflow  com- 
prises the  stream  of  total  slime  going  to  the  agitation  system.  Its  under- 
flow is  fed  into  a  second  duplex  Dorr  classifier,  the  overflow  from  which 
returns  to  the  cone  and  overflows  from  there  into  the  treatment  circuit, 
while  the  sand  product  is  taken  to  the  Frenier  pump  which  elevates  it,  for 
further  grinding,  into  the  small  tube  mill.  It  will  be  noted  that  the 
rather  complicated  flow  of  pulp  through  the  grinding  and  separating 
machines  is  merely  an  exceptionally  careful  system  of  separating  sand  and 
slime  so  that  the  pulp  for  agitation  may  be  uniformly  and  finely  reground. 

Agitation  of  Slime. — A  10X30-ft.  Dorr  thickener  receives  the  slime 
pulp  now  prepared  for  agitation,  thickens  it  to  a  consistency  of  about  1 : 1 
and  delivers  the  thickened  product  to  a  battery  of  four  38  X  10-ft.  Pachuca 
tanks  which  are  operated  in  series.  During  agitation  treatment  the 
solution  is  maintained  at  3  Ib.  KCN  per  ton  and  2  Ib.  CaO  per  ton.  Due 
to  the  character  of  the  ore,  the  chemical  consumption  of  cyanide  is  so 
small  that  it  may  be  disregarded.  The  total  cyanide  loss  is  J  Ib.  per  ton  of 
ore  milled  and  is  almost  entirely  mechanical.  The  lime  consumption  is 
about  J  Ib.  per  ton  of  ore  treated. 

Discussion  of  Filtration. — Pulp  from  the  agitation  tanks  is  received  in 
another  10X30-ft.  Dorr  thickener  which  delivers  thickened  pulp  into  a 
series  of  stock  tanks  which  are  equipped  with  mechanical  agitators,  in 
which  the  pulp  is  kept  thoroughly  mixed  until  ready  for  filtration.  The 
overflow  is  returned  to  dilute  the  inflow  into  the  first  thickener,  the  solu- 
tions being  disposed  of,  as  shown  in  the  accompanying  flow  sheet.  A  48- 
leaf  filter  of  the  stationary  vacuum  type,  or  Butters  system,  is  used  for 
filtration,  and,  although  it  has  proved  satisfactory,  it  will  probably  be  re- 
placed by  continuous  decantation  in  the  near  future,  experience  and  experi- 
ments having  shown  that  system  to  be  more  economical  and  quite  as 
efficient.  Tailing  from  the  filtering  tanks  is  discharged  by  gravity  into  a 
dam,  where  the  material  is  skimmed  at  intervals,  as  has  already  been 
described.1 

Solutions  from  the  filtration  plant  are  precipitated  and  returned  to  a 
treatment-solution  storage  tank,  from  which  the  solution  for  treatment  is 
drawn.  Any  surplus  at  this  point  may  be  returned  to  the  mill-solution 

1  "Eng.  and  Min.  Journ.,"  Mar,  22,  1913;  p.  618. 


TWO  ARIZONA  MILLS 


207 


Mill  Solution 
Tank' 


Q- ',. 


I!    -} 


Overflow 


Ore  from  Mine 

No.  2  Austin  6yrcttory  Crusher 

IJ-tfRobins  BettConveyor 
Mill  Bin 

4- Automatic  Feeders 

20, 1250-1  b.  Stomps  105  Drops . 
6" Drop.  12-Mesh  Screen 

6' Hydraulic  Cone 
1. 5x22' Tube  Mil  I 
I- Duplex  DorrCtassifier 

1-8x54  'Frenier  Pump 
\    I- 5^16' Tube  Mill 

I 

l-5'Hydrau/ic  Cone 

I-  Duplex  Dorr  Classifier 

Pumps 

l-10x$0  Dorr  Thickener 


P 


.     4-38xlO'Pachuca  Tanks 
!  Series  System 

*     HOxSO' Dorr  Thickener 
rflo* 

3-27x5  'Stock  Tanks,  Mechanical 
Agitators  8  K.p.m. 

I,  46-Leaf  Butters  Filter 
Slime  Residue, 


2  Clarifying  Tanks 
Zinc-dust  Feeder 
Pumps 


Pmc'ipitate\ 


Pumps 


FLOW  SHEET  OF  TOM  REED  MILL. 


208 


DETAILS  OF  CYANIDE  PRACTICE 


tank  at  the  head  of  the  mill,  thus  maintaining  a  solution  balance  without 
difficulty. 

Precipitation  Method. — Zinc  shavings  have  been  used  for  a  long  time 
for  precipitating  the  gold  from  cyanide  solutions  at  the  Tom  Reed  mill, 
but  recently  the  system  was  changed  and  the  Merrill  zinc-dust  apparatus 
installed.  The  system  gives  entirely  satisfactory  results,  420  tons  of 
solution  being  precipitated  in  a  plant  designed  to  handle  only  300  tons. 
With  zinc  shavings,  the  consumption  of  zinc  amounted  to  0.25  Ib.  per  ton 
of  ore  treated,  while  with  zinc  dust  only  0.15  Ib.  is  necessary. 

The  mill  crushes  an  average  of  150  tons  per  day,  requires  134  hp.  for  its 
operation,  and  employs  16  men  for  its  regular  control.  Pebble  consump- 
tion in  the  tube  mills  amounts  to  4  Ib.  per  ton  of  ore  crushed.  While 
itemized  cost  data  is  not  obtainable,  the  aggregate  is  $2.50  per  ton  milled, 
including  the  depreciation  account.  Average  extraction  is  95%  of  the 
gold  in  the  ore. 


THE  VULTURE  MILL,  WICKENBURG,  ARIZ. 

The  Vulture  Mill.— The  property  of  the  Vulture  Mines  Co.  is  situated 
about  14  miles  southwest  of  Wickenburg,  in  central  Arizona.  The  coun- 
try is  practically  desert,  and  water  is  scarce.  There  is  no  available  wood 
for  fuel,  and,  as  coal  is  extremely  expensive  laid  down  at  the  mine,  distil- 
late and  oil  are  used  entirely  to  furnish  motive  power.  Power  for  the  mill 
is  furnished  by  two  150-hp.  Nash  engines,  which  are  entirely  satisfactory. 

The  valuable  metal  is  gold,  which  occurs  in  white  quartz,  together 
with  pyrite  and  small  quantities  of  galena  and  chalcopyrite,  sometimes 
banded  and  sometimes  occurring  in  masses  with  the  pyrite.  The  present 


TWO  ARIZONA  MILLS  209 

orebody  is  one  which  was  discovered  in  1911,  the  vein  having  been  faulted 
and  the  ore  entirely  lost.  The  mine  is  an  old  one,  having  been  discovered 
by  H.  Wickenburg  in  1863.  Surrounding  country  rock  is  dioritic  schist, 
no  igneous  intrusions  having  any  influence  on  the  ore  deposition.  The 
average  milling  ore  contains  gold  to  the  value  of  approximately  $20  per 
ton.  Mine  ore  is  trammed  a  short  distance  from  the  working  shaft  to  the 
crusher  bins,  from  which  it  is  fed  to  a  gyratory  crusher.  The  crushed  ore, 
which  passes  a  2-in.  ring,  drops  to  a  conveyor  belt,  and  is  elevated  to 
another  ore  bin.  From  this  the  ore  is  hoisted  over  a  trestle  700  ft.  long 
to  the  mill  bins  by  means  of  a  friction  hoist  on  the  mill  line  shaft. 

Regrinding  in  Pans. — The  stamp  installation  consists  of  twenty  1600- 
Ib.  stamps,  dropping  98  times  per  minute  through  6J  in.,  and  crushing 
through  a  40-mesh  round-aperture  screen.  Chuck  blocks  are  used,  and 
the  ore  is  subjected  to  amalgamation  inside  the  mortars. 

A  Dorr  duplex  classifier  follows  the  stamps,  and  separates  the  pulp  into 
sand  and  slime.  Solution  is  used  throughout  the  mill,  including  the  stamp 
crushing  department.  The  slime  portion  of  the  ore  is  elevated  to  three 
Deister  slime  concentrators,  the  tailing  from  which  is  taken  to  a  series  of 
thickeners.  The  classified  sand  is  reground  in  two  5-ft.  Wheeler  pans, 
which  make  42  r.p.m.,  and  each  is  capable  of  grinding  35  tons  per  day, 
85%  of  which  passes  a  100-mesh  screen.  The  consumption  of  iron 
amounts  to  about  one  pound  per  ton  of  ore  treated. 

Continuous  Decantation  without  Filtration. — From  the  two  grinding 
pans,  the  ground  pulp  passes  to  four  Wilfley  concentrators,  the  high-grade 
sulphide  being  here  removed,  and  the  tailing  from  the  tables  is  again 
reground  in  a  third  Wheeler  pan,  identical  with  the  other  two.  The  prod- 
uct of  this  third  pan  is  sent  directly  into  the  first  of  a  series  of  continuous 
thickening  tanks,  together  with  the  slime  from  the  Deister  concentrators. 
In  this  series  of  thickeners,  there  is  continuous  decantation  and  thickening 
of  pulp,  followed  by  a  thinning  by  means  of  weaker  solutions.  Water  for 
final  washing  enters  the  last  thickener  and  progresses  to  the  first  in  a  direc- 
tion contrary  to  that  of  the  moving  pulp.  The  accompanying  flow-sheet 
diagram  shows  the  movement  of  the  pulp  and  solutions  in  the  thickener 
series,  as  well  as  in  the  rest  of  the  mill.  No  filter  is  in  use  and  the  final 
thickener  is  required  to  discharge  the  pulp  at  the  maximum  possible 
thickness.  It  will  be  noted  that  there  are  no  tanks  for  agitation  of  the 
pulp,  the  operation  being  practically  grinding,  concentrating,  washing 
and  discharging.  The  operators,  however,  believe  that  better  results 
could  be  obtained  by  using  one  or  two  agitation  tanks  ahead  of  the  thick- 
ener series,  and  such  tanks  will  probably  be  installed  in  the  near  future. 

Zinc  dust  is  used  as  a  precipitant,  a  feeder  of  the  belt  type  being 
installed.  A  precipitate  press  of  the  Merrill  triangular  type  having  sixteen 
36-in.  frames  is  used  to  collect  the  precipitate,  which  is  not  acid  treated, 


210 


DETAILS  OF  CYANIDE  PRACTICE 


but  is  melted  directly  in  an  oil-fired  tilting  furnace  and  the  bullion  shipped. 
Slags  formed  in  the  melting  are  periodically  smelted  in  a  small  blast  fur- 
nace, the  lead  bullion  cupelled  and  the  product  added  to  the  regular  output 
of  the  mill. 

It  is  of  particular  interest  to  note  that  in  the  two  modern  and  efficient 


battery 'Solution 
'Tank 


Gyratory  Crusher 
Be/ f  and  Tram  to.fli/l 

Mi  I  I  Bin 
Challenge  Feeders 

20- 1600-1  b.Stamps 

98  Drops,  6j  "Drop,  40- Mesh  Screen 

l-Duplex  Dorr  Classifier 
2~  5'  WheeJerGrind/ng  Pans.  42  R.p.m. 

4-Wilfley  Concentrators 
I-  5 '  Wheeler  Grinding  fan ,  42  Rp.m. 


4 -Dorr  Thickeners 

l-2B'xl5' 

3-32x12' 

Continuous  Counter-current  Decantation 


Wafer, 


\Slime  fesf'afi/e 

line-dust  Feeder 
Precipitate  Press 

Oil-  fired  Tilting  Furnace 
jllfon 

FLOW  SHEET  OF  VULTURE  MILL. 


plants  described  here,  experiment  has  shown  that  expensive  filtration 
plants  may  be  considered  superfluous.  At  the  Tom  Reed  mill,  where  a 
filtration  plant  was  originally  installed,  it  will  be  superseded  by  contin- 
uous decantation,  resulting  in  a  material  economy,  while  at  the  Vulture, 
no  filter  has  been  installed,  the  series  of  four  thickeners  satisfactorily  doing 


TWO  ARIZONA  MILLS  211 

the  work.  In  practice  the  thickeners  deliver  a  product  which  is  accep- 
table from  the  point  of  view  of  contained  moisture.  The  first  of  the  series 
at  the  Vulture  mill  delivers  a  product  containing  31%  moisture;  the 
second,  43%;  the  third,  42%;  and  the  fourth  and  final  one,  35%  mois- 
ture. If  necessary,  the  thickener  can  deliver  a  product  containing  as 
little  as  25%  moisture  on  a  slime  which  settles  rapidly.  It  is  not  too 
much  to  say  that  on  the  same  material,  the  vacuum  filter  cannot  do  a 
great  deal  better  than  the  thickener,  considering  all  points  at  their  real 
value.  In  the  statements  that  are  made  here  in  this  connection,  reference 
is  made  solely  to  the  treatment  of  gold  ores  of  milling  grade.  Silver  ores, 
requiring  solution  of  greater  cyanide  strength,  may  be  treated  by  a  varia- 
tion of  the  system  which  will  be  mentioned  in  a  later  article. 

Extremely  fine  grinding  is  not  required  on  the  Vulture  ore  to  secure 
the  economically  maximum  extraction.  Classification  stated  in  the 
accompanying  table  shows  the  condition  of  the  pulp  undergoing  treat- 
ment. 

CLASSIFICATION  OF  SLIME  PULP 

+  80  mesh 12.0% 

+  100  mesh 2.5% 

+  150  mesh 23.5% 

+200  mesh 46 . 5% 

-200  mesh 45.5% 

Solution  is  used  throughout  the  mill,  usually  maintained  at  about  2.1 
Ib.  KCN  per  ton  of  solution.  Lime  is  added  at  the  mortars  in  sufficient 
quantity  to  maintain  an  alkaline  condition. 

About  1  Ib.  of  KCN  is  consumed  in  treatment,  2  Ib.  of  lime  and  0.4 
Ib.  zinc,  in  the  form  of  zinc  dust.  Costs,  while  not  at  present  available  for 
publication,  are  reasonable  in  view  of  the  adverse  conditions  already  men- 
tioned. These  Arizona  plants  are  particularly  instructive  as  examples  of 
first-class  operation  under  difficulties. 


INDEX 


Acid  treatment  of  precipitate,  59,  61 
Additon,  A.  Sidney,  144 
Agitation  methods,  7,  33,  43,  56,  71 

system,  106,  111,  116,  120,  133,  164, 

176,  190,  191,  206 
Ajax  mill,  81 

Akins  classifiers,  70,  157,  171 
Aluminum  as  precipitant,  9 
Amalgamation,    15,    18,    19,   24,   31,   42, 
69,  177 


B 


Belmont  mill,  105,  125,  130 
Bismark  mill,  62 
Black  Hills,  Practice  in,  50 
Black  Oak  mill,  188 
Blue  Flag  mill,  81,  97 
Bromocyanidation,  88' 
Butters,  Charles,  15,  23 
Buffalo  mine,  cobalt,  2,  6 
refining  furnace,  11 
Butter's  filter,  5,  10,  17,  57 
discharge,  126 


Cadogan,  A.  G,  140 

Calculation  of  extractions,  140,  142,  144, 

146,  155 

Central  mill,  177 
Chilean    mills,    53,  56,  60,  85  123,  163, 

171 

Churchill  Milling  Co.,  161 
Clevenger,  G.  H.,  15,  23,  94 
Cobalt  District,  Ontario,  1 

ore  character,  1,  14 
Concentrate  treatment,  31,  183,  186 

of  slime,  106 
Concentration,  31,  86,  109,  134,  157,  180, 

209 

Continuous  decantation,  96,  209 
Cost  of  cyanidation  at  Tonopah,  125 


Costs  of  milling,  10,  77,  83,  156,  164,  184, 
193 

of  shipping  precipitate  and  bullion, 

92,93 

Cripple  Creek,  Colo.,  79 
Crushing  in  solution,  136 

advantages  and  drawbacks,  130 

Tonopah  ores,  139 
Cunningham,  Noel,  38 
Cyanide,  consumption  of,  8 

at  Nipissing  mill,  17 

Liberty  Bell  mill,  75 

Stratton's  Independence,  89 

West  End  mill,  109 

Montana-Tonopah,  113 

Tonopah  Extension,  119 

MacNamara,  122 

Nevada  Hills,  156 

Nevada  Wonder,  165 

North  Washington  P.  &  R.  Co.,  173 

San  Foil  mill,  176 

North  Star,  184 

Black  Oak,  192 

Gold  Road,  200 

Vulture,  211 


1) 


Decantation  washing,  158 
Desert  mill,  122 
Dewey,  Frederick  P.,  95 
Dome  mill,  Porcupine,  39 
Dominion  Reduction  Co.,  2 
Dorr  J.  V.  N.,  59 
agitators,  31,  158 

classifiers,  42,  54,  60,  97,  103,  111, 

114,  120,  164,  176,  190,  197,  206 

thickener,  32,  33,  42,  54,  60,  71,  97, 

108,  120,  158,  165/176,181,190, 

•  197,  206 


E 


Empire  mill,  184 

Extension,  Tonopah,  mill,  114 


213    . 


214 


INDEX 


Extraction  percentage,  8,  45,  52,  58,  76,  Linings  for  tube  mills,  132 

89,  109,  113,  117,  122,  135,  140,  Lundberg,  Dorr  &  Wilson  mill,  59 

142,    144,    146,    155,    165,    174, 

176,  184,  192  M 


Faber  du  Faur  furnace,  109 
Filter,  Butter's,  111,  117,  120,  199,  206 
Filters,  pressure,  Merrill,  10 
vacuum,  108 

Butters,  5,  10,  17,  57,  88 

Moore,  36,  54,  59,  71 


G 


Golden  Reward  mill,  53 

Goldfield  Consolidated  mill,  131 

Gold  Road  mill,  195 

Grading  analysis,  16,  71,  84,  85,  100,  116, 

211 
Grass  Valley,  Calif.,  177 


II 


Hardinge  conical  mills,  5,  7 
Hendryx  agitators,  71,  111,  133 
Hollinger  mill,  Porcupine,  28 
Homestake  mill,  50,  62 


MacNamara  mill,  119 
Mechanical  agitator,  136 
Merrill  filter,  10,  44 

precipitation  press,  5,  37,  44,   183, 

186,  200,  209 

Metallurgical  history  of  Republic,  Wash- 
ington, 168 
Minnesota  mill,  66 
Montana-Tonopah  mill,  109 
Moore  filters,  10 
Mother  Lode  ores,  Calif.,  188 


X 


Nevada  Hills  mill,  153 

Wonder  mill,  161 
Newton,  H.  W.,  170 
Nipissing,  mine,  Cobalt,  1,  5 

grading  analysis,  16 

high-grade  mill,  14,  23 
North  Star  mill,  177 

North  Washington  Power  and  Reduction 
Co.,  170 


Irvin,  Donald,  F.  126 
J 

Johnston,  James,  15 
Jones,  A.  H.,  136 


K 


Kirby,  A.  G.,  38 


Leaching,  51,  58,  87,  123,  180 

Lead  salts,  use  of,  8,  75,  109,  113,  119, 
122,  137,  156,  165,  173,  192 

Liberty  Bell  mill,  67 

Lime,  use  of,  7,  44,  52,  75,  84,  85,  109, 
111,  113,  119,  122, 137,  156,  160, 
165,  173  176,  184, 192,  202,  211 


O 

O'Brien  mill,  Cobalt,  3 
Oliver  filters,  160,  165,  181 
Ophir  mill,  97 
Ovoca  classifier,  86 


Pachuca  tanks,  7,  43,  56,  133,  158,  164, 

181,  186,  197,  206 
Parral  agitator,  7 
Pans,  regrinding,  209 
Parsons,  A.  R.,  125,  139 
Porcupine,  Dorr  mill,  39 
Hollinger  mill,  28 
ore  character,  28  ,39 
Portland  mill,  81 
Precipitation  systems,  9,  18,  25,  37,  57, 

59,  73,  88,  109,  113,  122,  160, 

173,  183,  199,  208 


INDEX 


215 


11 


Refining,  18,  19 

furnace  at  Buffalo  mill,  11 
Reid,  J.  A.,  49 
Reliance  mill,  62 
Republic,  Washington,  168 
Reverberatory  furnace,  12,  21 
Robbins,  P.  A.,  38 
Roll  crushing,  51,  55,  60,  66,  138,  139,  171 


Tonnage  estimations,  46,  52,  74 
Tonopah  ores,  of,  102 

practice  at,  102  to  152 
Treatment,  cost  of,  10 
Trent  agitators,  33,  36,  106,  116,  120,  133, 

158,  172 
Trojan  mill,  55 
Tube  mills,  6,  16,  24,  29,"42,  70,  103,  114, 

120,    132,    163,    172,    182,    190, 

197,  205 


S 


San  Foil  mill,  174 
Settling  area,  186 
Shipping  precipitate,  89,  90 
Simpson,  George,  Jr.,  142 
Smelting  precipitate,  46,  73 
Smuggler-Union  mill,  70 
Solutions,  27 

heated,  74,  111,  117,  135,  136 

progress  of,  99 
Sorting  ore  at  Tonopah,  129 
Stamps  and  Chilean   mills,  comparison 

of  lost  time  in,  163 
Stratton's  Independence  mill,  81 


Vacuum  pump,  73,  108 
Victoria  mill,  58 
Vulture  mill,  208 

W 

Wasp  No.  2  mill,  51 

Watson,  R.  B.,  23 

Welton,  Wm.  S.,  146 

West  End  mill,  103 

Wheeler,  K.  T.,  89 

Williams'  hammer  trommel  mill,  174 

Wonder,  Nevada,  mill,  161 


Temperature  of  solutions,  9 
Thickness,  32 

Tigre,  El.  Yzabal,  Sonora,  126 
Tom  Reed  mill,  203 


Zinc  dust,  as  precipitant,  9,  44,  113,  160, 

173,  183,  186,  199,  208,  209, 
shavings,  9,  18,  52,  57,  59,  73,  88, 
109,  122 


293556 


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